The show has plenty of these, which is impressive for a gag-run comedy where Status Quo Is God:
- Once the basis for one of Doofenshmirtz's Evil Plans in "Swiss Family Phineas" when he remembers what "Big Laundry" meant.
- Which doesn't make sense seeing as how in the credits of "The Lake Nose Monster" he remembered that it meant that it was laundry day, complete with a humongous pile of clothes. Maybe it had a double meaning?
- Lindana references are pretty common throughout the series. Word of God says that's the creators' "favorite thing" about Linda.
- In one episode, Monogram tells Perry that the agency has been tracking Doof's online activity, and found out what he is getting Perry for Christmas (a vase). In the Christmas episode (a full season later), Doof gives Perry that same vase.
- In his youth, Doof was forced to stand in the front lawn of his family's home as a lawn gnome, since they lost the real one. Doof then references the same story (with the same background) in "The Chronicles of Meap", commenting, "...you remember that backstory, Perry the Platypus." He does this in "Got Game?" as well, which is odd considering he thought he was talking to some dog.
- Doofenshmirtz: Life was simpler back when I was a garden gnome.
- Unless you consider he was talking to the audience.
- In several episodes characters are seen humming or singing along to either "Gitchee Gitchee Goo" or "I'm Lindana and I Wanna Have Fun", true to the one-hit wonders that they are, and they're background music ridiculously often.
- As mentioned in its initial episode—"Flop Starz"—a tinny muzak version of either one of these songs is played any time anybody rides an elevator.
- Brick, brick, brick, briiiick, brick, brick, brick, it's fun! In another episode, Doofenshmirtz gets angry about a billboard advertising these that now is blocking his view.
- In "Does This Duckbill Make Me Look Fat?", Doofenshmirtz comments that he really should have unplugged his Shrinkinator before using it as a planter. In "Let's Take a Quiz!", he still hasn't unplugged it, and it's covered in sticky note reminders to unplug it.
- In "Got Game?", when Doofenshmirtz is buying a dog (Perry in a bad disguise) he tells the pet store owner that he'll pay any price since he gets a huge alimony check every month. Several episodes later his ex-wife Charlene comments that she must be paying WAY too much alimony if he can afford to build a massive ice-cream-sundae-making machine(inator).
- In the second half of "Phineas and Ferb's Hawaiian Vacation", Doofenshmirtz gets stranded on a deserted island and comments on how he wishes he had a BO-AT.
- Doofenshmirtz: Or "boat", as I've now been told how it’s been pronounced..
- Heck, this might as well be Once an Episode. Every episode has its own 'continuity' section on the Phineas and Ferb Wiki, and they're generally rather hefty.
- In "Swiss Family Phineas", Candace questions how come Phineas and Ferb didn't make something to get them off the island and even suggests how they should have done it, including using a giant slingshot to launch them off the island. In "Summer Belongs to You", they use a slingshot to launch a giant paper plane so they can leave this island.
- Two in "Wizard Of Odd". First when Candace is being chased by flying squirrels and she screams "Squirrels, Squirrels!!" identically to how she sings it in S-I-M-P from "Comet Kermillian". Later, they play a parody of "BUSTED" (the song from "I Scream, You Scream") over the credits featuring the Tin-Man called "RUSTED".
- In "Brain Drain", Doofenshmirtz notes that his "De-Volitionator" is not to be confused with his "De-Evolutionator from a few schemes ago", as seen in "Phineas and Ferb's Hawaiian Vacation".
- Doofenshmirtz also mentions when Perry blew him up at Vanessa's birthday party, a reference to "Dude, We're Getting the Band Back Together!".
- In several episodes, Candace talks about marrying Jeremy and having kids named Xavier & Amanda. When Phineas & Ferb meet the future her in "Phineas and Ferb's Quantum Boogaloo", those are in fact the names of her kids, excluding the one named Fred.
- Taken to the extreme in "Rollercoaster: The Musical". "Mom Look" has Candace rattling off a list of the boys' previous schemes, and the finale "Carpe Diem" features almost every character in the whole show at that point. Just a few examples are the guy with the wolf puppet from the cattle episode, the anime girls from "Summer Belongs to You", Mishti from "That Sinking Feeling", and Doofenshmirtz's childhood pal Kenny.
- In the same episode, when Candace come out of the grocery store, the two adult Candaces from Quantum Boogaloo are hiding behind a car.
- For more details see Continuity Cavalcade above.
- In the episode "Fly On the Wall", Phineas asks if they know what summer is about, and so the others begin to sing Summer (Where Do We Begin?) What made this one so epic was that everyone had remembered the song even after the memory wipe- despite having no idea how.
- In "Phineas and Ferb Save Summer", when it shows outer space, you can see the Tower of Danville from "The Doof Side of the Moon."
- In "The Chronicles of Meap", Dr. Doofenshmirtz mentions having to stand in for his family's lawn gnome, as he mentioned previously in "Lawn Gnome Beach Party of Terror!", and even tells Perry the Platypus "You remember that backstory..."
- In "De Plane! De Plane!", Candace gets jealous when she sees Jeremy talking to another girl at the pool party, and Stacy references the events of "S'Winter" in warning her not to get carried away.Stacy: Slow down! Remember how you thought Jeremy was with a girl, and she wasn't even a she? And then you ended up on that snowboard, all crazy and out-of-control?
- In "Invasion of the Ferb-Snatchers", Candace freaks out when she thinks Ferb has been replaced by an alien doppelganger, and goes to hide in the panic room first seen in "I, Brobot".
- In "We Call It Maze", Isabella's mentoree Melissa recognizes Candace as the girl who earned fifty accomplishment patches in one day, as seen in the episode "Fireside Girls Jamboree".
- In "Bullseye," Linda mentions that lip-syncing isn't her only talent.
- "Bullseye" also features Doofenshmirtz in his underwear skating into a toilet, a nod to the viral video in "Tip of the Day".
- "For Your Ice Only" features Isabella introducing Hockey Z-9 in her deep announcer voice from "It's A Mud, Mud, Mud, Mud, Mud World", even ending with the phrase "Thanks, Dink!" like she did in the original sequence.
- During "When Worlds Collide", Linda reminds Candace that earlier in the day, she tried to show her a giant ant farm, referencing "Gi-Ants"
