Dr. Simon Finch-Royce (John Cleese) of "Simon Says" is a famous marriage counselor. After describing his reputation for successful curing of marriage problems, Diane notes that rumor has it he even counseled Prince Charles and Princess Diana. Uh-huh. Considering what happened between that famous couple, soon after...in hindsight, this can easily be seen as something of an accidental indication that maybe Diane, after hearing his "advice", is on to something—Dr. Finch-Royce really is highly overrated.
Cliff has used the phrase "Hit the bricks!" twice in the series: once when Sam proved to another patron that he could still do his mug sliding trick (where the mug hooks a right at the corner of the bar) and once when a patron asked him to give him a jump start in a fierce rainstorm. The second time turned out to be a dream Diane was having. So, how did she remember this once-used phrase for her dream? Well, between the two episodes, Cliff tried to scare off another patron who'd been bullying him by pretending to know karate. He demonstrated his "skill" by literally hitting a brick and Diane was the one who had to take him to the hospital afterwards. Evidently, this incident caused her to remember the previous incident and it stuck with her all those years.
Speaking of the mug-sliding trick, Sam's ability to slide a mug of beer down the bar so that it makes the turn at the corner makes more sense when you remember he used to be a baseball pitcher. He's just applying the same skills he used to throw curve balls and such.
It's been mention time and again in-series, and shown once during the opening of Season 3, but Diane really is Sam's Morality Pet, and part of the Flanderization can be attributed to Diane not being there to rein him in. (It doesn't explain the complete forgetting that Sam's an alcoholic, but perhaps Diane effectively "cured" him of that problem, and he's sublimating his desire to drink into sleeping around even more.)
After Robin Colcord is arrested and starts working on a road crew, Sam teases Rebecca by playing Bobby Fuller's "I Fought The Law". It seems like simple teasing at first, but if you're familiar with the song, it should cause much angst for Rebecca. The lyrics state that the narrator's relationship with his girl ended because of his incarceration.
Furthermore, Rebecca tries to play Tammy Wynette's "Stand By Your Man" only to discover Sam rigged the jukebox to play "I Fought The Law" no matter what song is selected. There's a hint that Sam rigged the jukebox the first time "I Fought The Law" plays. The song starts with the lines where the title is sung while there's actually a brief intro and another line before that happens.
When the gang hears that the local drive-in theater is closing, they decide to pay it one last visit. Norm asks not to be sent to the snack bar because he racked up a large tab and cites the fact that they're closing as evidence. Although if you subscribe to the theory that movie theaters make most of their revenue at the snack bar, you might not think Norm was joking.
Just after Frederick Crane is born, Frasier stops by the bar and makes a joke. ("How many babies does it take to change a light bulb? None. Babies lack the motor skills and depth perception to change a light bulb.") Most (maybe all) of the humor of this joke is how overly complicated it is with the other characters simply finding it a bad joke. In other words, the new dad just made a dad joke in his own style.
One episode has Kevin McHale of the Boston Celtics visit the bar for Norm's birthday and get involved in one of their discussions. Specifically, it's the one where they ask themselves how many bolts there are in the floor of Boston Garden. McHale becomes obsessed with finding out the answer which distracts him during a game where he tries counting the bolts while playing. Carla really lays into the guys, calling them vampires at one point for spreading their obsessions. But there's possibly another layer to her comment: obsessive counting is a common trait in vampire folklore and someone as superstitious as Carla would definitely know that.
Boston was the third or fourth choice for the location of the series, behind Barstow, California, Kansas City, Missouri, and possibly Chicago. In retrospect, Boston was by far the best choice considering that it’s both a hotbed of East Coast intellectualism as well as being a hardcore sports town.
The gentrification and corporate takeover themes of the Rebecca years can be seen as an unconscious commentary on the show's rise from low-rated "cult" show to a ratings powerhouse.
In the episode A Bar Is Born, Woody says he had Sam as a crossword answer for “Former Red Sox Star” but the clue had fifteen letters. “SAM MALONE” doesn’t have fifteen letters, but “CARL YAZSTREMSKI” (whom Sam occasionally resented for being more popular than him) does.
When people are speculating that Cliff might be gay, Coach brings up Duke Roberts, who displayed several stereotypes but wasn't gay. Sam calmly informs Coach that Duke was in fact gay. This gives Sam's attitude in "The Boys In The Bar" more nuance. Sam is more shocked that his fellow womanizing buddy was living a lie than actually homophobic. He was accepting of the semi closeted Duke but upset when it was someone he knew well.
It's unclear whether Gary Portnoy and Judy Hart Angelo had seen the pilot when they composed the theme, but they picked up on the heart of the show with the chorus. What's the in-universe example of "everybody knowing your name?" "NORM!" The irony is that "NORM!" was an ad lib suggested by Nicolas Colasanto. A former hard drinker, Colasanto recalled that there was one regular at the bar whom everybody greeted enthusiastically.
In the episode The Boys In The Bar, take note of the barfly warning that letting gay guys in the bar will lead to "wild music and guys exchanging phone numbers." As it turns out, he's one of the gay guys Diane befriended (and who ends up kissing Norm).
When Sam and Woody have the contest to see who can kiss Rebecca first, one of Woody's attempts involves trying to hypnotize her. (Key word being "trying".) It kind of makes you wonder what about Woody's mindset would make him think this was okay and that he would still have a job afterwards. (Remember, Rebecca is Woody's boss at this point of the series.)
One of the "Bar Wars" episodes has the Cheers gang pull a prank where they effectively shut down Gary's on St. Patrick's Day, which the characters recognize as the biggest bar night of the year. At the end, it turns out that Gary had decided to take a vacation and his bar was already closed. While Cheers escapes retaliation for the prank, this is still bad news from a business standpoint. Gary would have to be doing very well if he could afford to be closed on the biggest bar night of the year.
Watching the third season it’s clearly visible just how sick Nicholas Colasanto was by then. It’s not exactly Harsher in Hindsight, as Coach is not that different in-universe, but it’s very sad to see him physically diminished.
Given that he finally realizes he has a problem with sex addiction, Sam's descriptions of sexual activity at a young age seem more like abuse in hindsight.
Diane's only known long-term relationships are Sumner Sloane (her professor), Sam (her boss), and Frasier (her psychiatrist when they started). It's possible Diane has never had a healthy appropriate relationship in her life.
While Carla turns out to be right to suspect Eddie of infidelity, her treatment of him (he describes her slamming his head on the refrigerator door) borders on abuse.
