Glenn tells Rick after rescuing him, that he puts his neck on the line for others in the admittedly naive hope that they'd do the same for him one day. Rick rescues Glenn two episodes later.
In the same span of time, Rick is unable to save Glenn from being abducted by the Vatos (though it later turns out Glenn was never in any danger as the Vatos were peaceful people putting up a tough guy front). In the Season 7 premiere, Rick is unable to save Glenn as he's beaten to death by Negan, and later explicitly laments that even though Glenn saved his life at the beginning, he couldn't save him now.
In the first episode, before the outbreak, we see two crows eating a dead animal in the road. Later, after the outbreak, there are two crows eating a dead human in Atlanta.
Before leaving King County for Atlanta, Rick gets some advice from Morgan when dealing with walkers: "They may not seem like much one at a time. But in a group, all riled up and hungry? Man, you watch your ass." Upon arriving in Atlanta, Rick saw that Morgan was right.
In the Season 1 finale, Rick tells Jenner he's grateful that the doctor is letting them go live their lives in the outside world. Jenner sadly says that "the day will come when you won't be." The title of the Season 7 premiere is exactly that phrase, and given the horrific deaths, psychological and physical torture Rick and the group endure, it's safe to say that the day Negan arrived was finally the day Rick agreed with Jenner.
Season 2
In the Season 2 opener, Grady Memorial Hospital is visibly seen while Rick is on a radio. The hospital becomes an important setting three seasons later.
In a deleted scene of the second season premiere, the group discovers the aftermath of an attack on the Vatos' camp that left no survivors. Andrea believes the place was overrun by walkers, but Daryl points out that they were all executed, professing he was more afraid of whoever did this than the walkers. In the second half of the season, the main outside threat is a large, hostile group of survivors, and from then on, the Big Bad's are always hostile people out for the group's blood.
In a flashback scene in "Bloodletting", Lori gripes to a friend of hers about an argument she and Rick had. She wishes that for once he would blow up at her. She gets her wish four episodes later when Rick flips out when he finds her 'morning after' pills.
Shane did remind Rick that even before the walkers rose, if 72 hours passed, then they as cops were no longer looking for a person, but the body. He used this as an example to illustrate the chances of finding Sophia alive were slim to none. At the end of "Pretty Much Dead Already", Shane was tragically right.
When Daryl come across Cherokee roses, he says that they will find Sophia soon. They do, though she is a walker. Additionally, when Daryl brings Carol the rose, he specifically says that the roses bloomed for the dead children.
While interrogating Randall in "Judge, Jury, and Executioner", Daryl learns of a group the former had belonged to that scavenged, raped, and murdered and were heading south. Three seasons later, we learn that Terminus (located near Macon, which is southeast of Atlanta) was taken over by a group of scavengers, rapists, and murderers.
The final shot of the second season features a prison, the same one they will take refuge in the next season and a half.
Season 3
In "Walk with Me", Merle tells Andrea that a gun in your face is just a greeting now, and he'd be more afraid of someone who walked up waving a white flag. At the end of the episode, this is exactly how the Governor approaches the soldiers from the helicopter, right before he and his men gun them all down.
A very subtle example lasting through all of Season 3: Early in the season, The Governor tells Merle that Woodbury would fall apart without him. The second half of the season has Merle joining Rick's group, and ultimately ends with everyone from Woodbury either dead or also joining Rick's group, aside from The Governor, Martinez, and Shumpert. It probably didn't help that Merle killed many members of Woodbury.
Season 4
In "30 Days Without an Accident", the last man to enter the store during the supply run pauses to look at the lower half of a corpse lying near the entrance. A tracking shot reveals its zombified upper body, and a bunch of other walkers, on the roof, which subsequently fall through onto the foraging party, just as the half-corpse's legs must have fallen from above.
When the veterinary college is searched in "Indifference", the camera pans over a pile of pathology books someone has left out along with makeshift camping supplies. Turns out there's a horde of plague-victim walkers on site, suggesting that a prior group of infected survivors tried to treat themselves using medications and textbooks they found there, only to succumb to their illness too quickly to benefit from their efforts.
The Governor and the Dolgen brothers come across a small group of survivors in "Dead Weight." Later in the episode, the group is slaughtered and looted by an unknown party. The Claimers, a group of murderers and plunderers, appear a few episodes later.
Lizzie alarmingly gains pleasure from nearly suffocating baby Judith in "Inmates". In the next episode she's center stage, she's killed her sister and planned to do the same with Judith.
Season 5
The Terminus survivors are seen being dominated by a black-haired, grinning psychopath in flashbacks of the season 5 premiere. Many fans wondered if the man was in fact an Early-Bird Cameo of Negan, to the point Word of God had to deny it. Despite this, the flashback ended up foreshadowing the plot of Season 7 when Negan did show up, as Rick's group was forced to submit to the black-haired, grinning psychopath and later rise up and defy him.
In "Four Walls and a Roof", Gareth snarls that Rick's group doesn't know what it's like to be truly hungry. Later in the season, during "Them", the group goes without food and water for far longer than they ever had and are dangerously close to succumbing to starvation and dehydration.
In "What Happened and What's Going On", Tyreese alone notices a discarded, broken clock. It's a symbol that his time is up, and he dies by the end of the episode.
Earlier in that same episode, the group walks past a human skeleton in the woods. Only one person looks at it: Tyreese.
In "Spend," shortly after Aidan starts a dubstep song in the van, the camera cuts over to Noah just as the singer in the song says "now you're going to die." Noah (and Aidan for that matter) both die near the end of the episode.
Season 6
Rosita and Spencer share a moment in "Now" as they observe the horde of walkers at Alexandria's gates. They end up sleeping together after Abraham cruelly dumps her later in the season and begin seeing each other.
Abraham personally takes it upon himself to monitor Sasha's well-being in Seasons 5 and 6, especially after she haphazardly slashed him in a murderous rage in "Them", and can sometimes be seen observing her from afar. He professes a newly developed crush on Sasha in "Always Accountable".
In "Start to Finish," we see several ants swarming around one of Sam's cookies, symbolizing how Sam would soon be devoured by several "ants" himself, which he is in the next episode.
Glenn's fate being bludgeoned to death with a baseball bat by Negan is alluded to several times throughout the earlier seasons before it finally comes to pass in "The Day Will Come When You Won't Be".
In "No Sanctuary", Glenn is almost beaten with a steel bat twice before it is interrupted.
In "What Happened and What's Going On", the camera lingers on a shot of Glenn picking up a baseball bat.
In "Not Tomorrow Yet", during the raid on the Saviors, Glenn finds a collection of Polaroid pictures of people's heads bashed in by something to the point they're bloody piles of mush. It ends up not only alluding to what Negan has in store for the group in the season finale, but ultimately and tragically Glenn's own fate when Negan picks him as a second victim after Abraham. We witness one Savior taking a Polarid of his remains, which are rubbed in Daryl's face by Dwight to taunt him for inadvertently helping get Glenn killed.
Season 7
In "The Day Will Come When You Won't Be", Rick has a vision of an impossible future in which his group is enjoying dinner together with the now deceased Glenn and Abraham, as well as Glenn's unborn child and Sasha, who's pregnant with Abraham's child. The last shot of the vision seemingly focused on Glenn and Abraham and how this vision will never come to pass. However, Sasha is also in the final shot. In the Season 7 finale, Sasha dies, meaning that last shot was telling us which major members of Rick's group would die in the season.
In "Sing Me a Song", Eugene warns Rosita about her suicide mission to kill Negan, stating that even if she was the risk taker, others may take the punishment if she fails. Come the midseason finale of Season 7, "Hearts Still Beating", where Rosita tries to shoot Negan, fails... and Olivia is killed by one of the Saviors when she refuses to tell Negan who crafted that bullet (as Negan had robbed Alexandria of all its guns). Eugene reveals that it was him, and is taken as a hostage for his useful bullet-crafting skills. In conclusion, two people suffered the consequences of that move rather than Rosita.
Season 8
In "Mercy", as Rick takes down a Savior, the latter mocks the former with this line: "Your kid's gonna die!". Seven episodes later, it comes true, as Carl reveals to Rick a zombie bite on his stomach.
Season 9
In "What Comes After", Rick suffers a hallucination of a massive field of his dead friends and family. The most prominently seen character is Jesus, who would die three episodes later.
In "Stradivarius", Carol says Daryl can't live out in the woods with "her" dog forever. This is not clarified until the extended Season 10 episode "Find Me".
Season 10
When Brandon praises Negan for escaping, the latter never confirms or denies that he escaped. "Walk With Us" and "Look at the Flowers" would confirm that he didn't escape - he was let out.
After Siddiq's death, Rosita fears being in the middle of the action because it could leave Coco without either of her birth parents to raise her. Her worst fear comes true in the final episode of the series, though she at least goes knowing she saved her daughter's life and Gabriel will be around to take care of Coco from there.
In "Find Me", Leah is shown to be an expert marksman, has numerous capabilities when it comes to survival, and refers to her lost family as her "squad", suggesting she is ex-military. Season 11's "Rendition" would confirm this when Leah returns and is revealed to be a loyal member of the Reapers, a violent gang of Afghanistan war vets.
Season 11
In "Acheron, Part II", Mercer has a subdued, but noticeable reaction when Eugene admits he is a virgin and was hoping Stephanie would be who he had his first time with. Pretty understandable given Mercer probably thinks it's TMI, right? "Rogue Element" and "The Lucky Ones" will reveal that Mercer is "Stephanie's" older brother, so Eugene just told his prospective girlfriend's brother that he wants to have sex with her!
In "Out of the Ashes", it's odd that Stephanie claims that it's a long, grueling process to be able to use the radio when she had spent hours upon hours on it with Eugene. We also see Eugene encounter a woman taking his favorite, rocky road ice cream, who becomes evasive and looks at Stephanie. "Rogue Element" and "The Lucky Ones" will reveal the woman is the actual "Stephanie" who used an illegal homemade radio to contact Eugene without restriction.
Also in "Out of the Ashes", Rosita tells Judith her mother died when Rosita was "a little older than you are now." This foreshadows Rosita's fate in the series finale, as she dies a few months after her conversation with Judith takes place, when Judith is "a little older" than she was at the time.
In “New Haunts”, Pamela evades Connie’s question about the class divide in the Commonwealth and instead introduces her to a citizen who won the lottery the year before. “A New Deal” reveals that Pamela handpicks the lottery winners who become members of the elite, suggesting that the lottery is just a way to reward people loyal to Pamela and turn them into good PR for herself.
