Both Marsh and Povenmire wrote for CatDog and Rocko's Modern Life. Both shows feature multiple minor characters with the name "Ferb."
At least three different cast members from The Rocky Horror Picture Show have starred or guest-starred in the series at least once. Richard O'Brien (Riff Raff) plays the father, Lawrence. Barry Bostwick (Brad MajorsASSHOLE!) plays Phineas's grandfather. And, Tim Curry (Dr. Frank N. Furter) guest-starred as a Mad Scientist in one episode. However, none of the characters nor their voice actors have interacted with each other as of yet.
As of "The Last Train to Bustville", Lawrence and Clyde, the grandfather, have had one, very awkward, conversation on camera.
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. Part of the reason is that Thomas Brodie Sangster records his dialogue in England while the rest of the main cast is in America.
Interesting parallel: Vanessa Doofenshmirtz is the non (or slightly less) evil child of a Harmless Villain, who constantly snarks at his parenting attempts, similar to Scott Evil. Fittingly, her boyfriend Monty is played by Scott's actor, Seth Green.
Accidentally Correct Writing: 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). Interviews have indicated that neither the show's creators nor Perry's voice actor were aware of this during production.
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 The 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.
Banned Episode: From 2009 to 2015, "Ready for the Bettys" was rarely if ever shown in reruns due to legal issues with a real life band called The Bettys which the writers did not know about at the time. This issue also caused "The Flying Fishmonger" (which was paired with it for syndication) and "Phineas and Ferb Musical Cliptastic Countdown" (which features the song "Ready for the Bettys") to be pulled as well.
Seemingly every post-"Busted" Vanessa sighting has resulted in her character gaining a song, apparently due to Disney realizing voice actress Olivia Olson has an excellent voice. She does the singing for a lot of other female characters as well— "I'm Lindana and I Wanna Have Fun", for instance, and in "The Monster of Phineas-n-Ferbenstein", she voices the backup singers. AND according to her Wikipedia page, she's also written songs for the show.
She also gets songs in several episodes in which Vanessa doesn't physically appear, such as the Christmas Episode. (The songs tend to be "no one's actually singing, just accompanying a montage" songs.) In addition, there are episodes where she does appear (e.g.: the Star Wars crossover) just for the sake of singing songs rather than to advance the plot. It helps of course that her father (Martin Olson) is one of the show's writers and does some of the music for it as well.
Vincent Martella (Phineas) and Ashley Tisdale (Candace) get their share of time in the musical sun as well, though of course, they are the main (regularly speaking) characters.
Let us not forget Alyson Stoner. Interestingly, at first it didn't look like this trope was going to come into effect for Alyson, since initially every song Isabella was involved in either kept her in the background or replaced her voice as a gag (S'Winter). However, as of the Christmas special she's been getting some really nice numbers and even sings the main part on two recent ones, and that's not even counting her rendition of "Let it Snow" from the Christmas album.
Carl's voice actor, Tyler Mann, is a talented singer, but he didn't get to show off his chops until "Where's Perry?" This is lampshaded by Major Monogram who states "You may have some cleverly hidden singing chops."
In one of the Halloween episodes, he's watching a horror movie and asks, "Where's all the rock and roll music?"
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. As of the middle of the fourth season, they've begun premiering on Disney XDagain and are aired on Disney Channel as part of an "Disney XD on Disney Channel" block.
Another oblique example of Channel Hop, the series originally premiered on Toon Disney and remained when the network was transformed to XD in 2009.
Also, the singing voices of female characters during a few songs are sometimes done by men.
Up to Eleven in the Japanese dub with the same character, since she's voiced by Taketora, aka Akuma.
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.
Descended Creator: Creators Don Povenmire and Jeff "Swampy" Marsh both voice the major characters of the show's recurring B plot; Heinz Doofenshirtz and Major Monogram, respectively.
DVD Commentary: "Attack of the 50 Foot Sister" and "The Chronicles of Meap: More Than Meaps the Eye" each have two: a standard one by Jeff "Swampy" Marsh, Dan Povenmire and Jon Colton Barry, and an in-character one by Dr. Doofenshmirtz and Major Monogram. The former episode's commentaries are on the DVD The Movie: Across the 2nd Dimension, while the latter episode's commentaries are on the DVD A Very Perry Christmas: Bonus Disc.
Executive Meddling: One of the best positive examples of this tropes. The producers of the show asked to have a musical in every episode. So far, many of them have been really catchy and memorable.
Disney also suggested to create characters that would become Isabella and Baljeet, both of which are quite popular.
One executive suggested that Perry should get a girlfriend, only for the creators to say that he is "married to his job". This was even made fun of in the episode "Nerds of a Feather".
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 jet-ski 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 NY Times panel had Dan and Jeff finally putting the bed the identity of Phineas's 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).
Long Runner: An interesting case due to Schedule Slip, though the series qualifies as one no matter what at least by typical Disney standards. If going strictly by season count, then Phineas and Ferb is still the longest-running series in Disney history with four seasons - but stuck in a multi-way tie in such with multiple other animated and live-actions series. When going by the actual length of time elapsed between the premiere of the first and last episodes, it's far and away the longest-running Disney series running through eight years from 2007 to 2015 and multiple Channel Hops (from Toon Disney to switching variously with Disney XD and Disney Channel).
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.
The full episode was aired again on Disney Channel on May 22, 2015 at 5:00am, and aired on Disney XD (for the first time) on June 10, during the marathon of every episode they aired in anticipation for the Series Finale.
For whatever reason, the Musical Cliptastic Countdown Hosted by Kelly Osbourne is not available on Netflix.
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.
In the Russian dub, both Phineas and Ferb had their voice actors replaced twice. Once for the second season, and once again for the third season and onwards. The differences between season 1 Phineas, then seasons 2 and 3 is very obvious. (Justified in a way: season one was dubbed in Moscow by SDI Media Russia, and the rest is done in St. Petersburg by Nevafilm)
Fireside Girl Holly was voiced by Cymphonique Miller from season 1 to the middle of season 2. After that, Diamond White would take over.
Fireside Girl Ginger (Stacy's sister) was voiced by Tiffany Espensen from season 1 to season 2. Michaela Zee would take over afterwards.
Real-Life Relative: Starting in Season 3, Ginger has been voiced by Michaela Zee, the daughter of Ming-Na Wen, who voices Dr. Hirano.
Schedule Slip: The show officially has four seasons - which have been stretched across eight years from 2007 to 2015. Going strictly by season count, it's in a multi-way tie with a number of other animated and live-action shows for longest running Disney series, but going by calendar year run it takes the prize by an almost insurmountable margin.
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 was to make them nine-year olds. 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.
Jeremy was originally going to be an unseen character, with Candace and others only referring to him in conversation. Likewise, Stacy was going to be just someone Candace spoke to on the phone.
Planty the Potted Plant was going to get more screentime, but most of his scenes ended up being cut for time.
Carl was a completely accidental character: one of the writers just started having Major Monogram make aside comments to the guy operating the camera. The other writers loved it and began including Carl in the show proper.
There was suppose to be a theatrical film of the series, but it fell through and the series was giving the usual Made-for-TV one. It's unknown if Across the 2nd Dimension was to be that movie.
The bugs from "Doof 101" and The O.W.C.A. Files can be seen among the early character designs, but it wasn't until "Doof 101" that they found a use for them.
Dr. Doofenshmirtz was originally going to be called Meddleshmirtz, and Candace was going to be blond.
Dan and Jeff wanted considered making Perry a capybara, before deciding on a platypus.
The original plot to "Tip of the Day" was very different. It would've been an elaborate 22 minute-long episode about a secret society that keeps the word "aglet" from becoming common knowledge to prevent the end of civilisation itself. Dan and Swampy both flatly rejected this idea, and it was redone as a much shorter and simpler story.
According to this article made back in 2011 at the height of the show's popularity, there were plans for a spin-off series involving The Fireside Girls; but it never got made.
Swampy Marsh's twitter confirms the spin-off episode "The OWCA Files" was a pilot for another spin-off series. They never heard back from Disney if the show was going to be picked up, so they moved on.
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.