Phineas and Ferb made a guest appearance in an episode of Extreme Makeover Home Edition in which they build a shrink ray to help Ty Pennington destroy a house. And yes, it is kickass.
Unlike most other shows, the cast does not record the scenes in the same room or even at the same time. Vincent Martella (Phineas) claims to have waited 2 years before meeting Thomas Brodie Sangster (Ferb) in person.
Accidentally Accurate: That chattering sound Perry makes? Platypi really do make those noises in real life (though of course not with their teeth, which they don't have).
Actor Allusion: Ferb's Precocious Crush on Vanessa Doofenshmirtz is a callback to the actors' previous roles in the movie Love Actually, where Thomas Sangster's character had a similar crush on Olivia Olson's character.
Candace's voice actress Ashley Tisdale is Photoshopped over Candace for her part in "The Curse of the Princess Monster".
When Candace competes against Wendy for a job at Mr. Slushy Burger, Wendy is voiced by Brenda Song, who worked with Tisdale on The Suite Life of Zack and Cody.
"Picture This" introduces Baljeet's uncle Maulik, named after Baljeet's voice actor Maulik Pancholy.
In "Phineas And Ferb Musical Cliptastic Countdown" the show was hosted by Major Monogram and Dr. Doofenshmirtz, who are both voiced by the creators.
In "Nerds of A Feather", Doof pitches a show to a man named Jeff MacGarland.
Which makes it even more funny to anyone who knows the creators well enough to get the "I got my own show!!" jokes. For those who don't, Doofenshmirtz's voice actor (the show's executive producer) used to work with MacFarlane on Family Guy.
In "Ladies and Gentlemen: Meet Max Modem!", Linda mentions that she only lip-synced "I'm Lindana and I Wanna Have Fun". The person who actually sings that song isn't the same person who voices Linda. (it's Olivia Olson, who voices Vanessa)
Richard O'Brien, who does the voice of the boys' dad, is best known for his role in Rocky Horror Picture Show and is very confused when the horror film he's watching has no rock and roll musical numbers.
Jenny and Django are also confirmed to be siblings.
Channel Hop: Sort of. From the second season onward, some new episodes premiere first on Disney XD as opposed to Disney Channel since the show skews more towards the former's demographic, and are then shown a week or so later on DC branded as "all new".
Well, sometimes a week or two. Sometimes six freakin' months.
As of "The Wizard of Odd", new episodes premiere on Disney Channel first again, but now they're labeled brand new when they premiere on Disney XD.
Dawson Casting: The main group of kids have ages 15 or under, according to the creators (with Candace being 15). None of their voice actors have ages falling into that range, with one of them [Maulik Pancholy] being in his late thirties. Incredibly, he's not even the oldest - the winner of that particular honour is Kelly Hu, who was 39 when she began voicing Stacy!
Jeremy and Vanessa were roughly the same age as their voice actors, Mitchel Musso and Olivia Olsen, respectively, during the casting.
Fandom Nod: In "What a Croc" the fan ship of Ferb/Gretchen got a brief nod after three nearly three seasons of extremely minimal interaction between the two characters. Although the nod itself was very brief, featuring both sharing a jetski after showing Phineas and Isabella quite happily doing the same.
I Knew It: "For Your Ice Only" confirms what many fans had been suspecting for a long time: Stacy and Ginger are sisters.
Jossed: A recent NY Times panel had Dan and Jeff finally putting the bed the identity of Phineas' real father...kind of. They admitted that it was not Doof (thus throwing a loop in many a viewer's theory on the real parents).
Missing Episode: "Ready for the Bettys" was pulled from the broadcast rotation in the United States after a band from New York with the same name thought they were copying them, according to one of the writers of that episode. This caused the episode it was paired with, "The Flying Fishmonger", to be removed as well. The episodes are still available on Netflix, iTunes, and Amazon Instant.
Name's the Same: The fact that Vanessa shares her first name with Vanessa Paradis might be why her boyfriend is named Johnny.
Non-Singing Voice: Played with in "Summer Belongs to You"; when the first song begins, we hear a voice that is obviously not Phineas singing as we pan up a body dressed in Phineas's clothes...and then we find out that it's Clay Aiken, dressed in Phineas's clothes. And then Chaka Khan dressed in Ferb's clothes enters.
Played straight with Ferb, as Danny Jacobs, Dan Povenmire and Aaron Jacob have done his singing voice.
One of Us: Dan Povenmire and Jeffrey Marsh are very aware of the large community/following that the show has. Marsh is a known contributor to the P&F Wiki, and they have confessed to at least being aware of this very site.
Real-Life Relative: Starting in Season 3, Ginger has been voiced by Michaela Zee, the daughter of Ming-Na Wen, who voices Dr. Hirano.
Screwed by the Network: Despite the exact opposite. Phineas and Ferb are subject to frequent marathons and specials across both of Disney's major channels, and they play a big part in a lot of the channel bumpers and advertising, but during the middle of Season 2 new episodes were subject to some severe Schedule Slip, to the point where they would air in other countries months before the US premieres, which are about a month apart from each other themselves. The premieres have since been getting more regular since the start of Season 3.
Shrug of God: The creators, when asked Phineas and Ferb's age; though the original pitch called them nine, this was retracted so that now they're merely "less than fifteen".
In "Quantum Boogaloo", however, they had to pick an age, so ten was given, as twenty years in the future Future Linda says that the boys are thirty years old. However, she may have been rounding; Word of God was that fans shouldn't necessarily take her literally.
"Act Your Age" takes place 10 years after the primary setting of the show, and Phineas and Isabella are getting ready to go off to college. This greatly suggests that they are only around 8-10 years old.
Word of Saint Paul: Long before Candace and Jeremy were an established couple, Jeremy's voice actor commented that Jeremy liked Candace back. Of course, it really wasn't all that hard to figure out, but still.