The show takes place in Danville, which is in Jefferson County which is in the Tri-State Area. Danville is named after creator Dan Povenmire and Jefferson County is named after co-creator Jeff 'Swampy' Marsh.
While other characters simply call him Perry, Dr. Doofenshmirtz always refers to Perry by his full name (Perry the Platypus). He doesn't abbreviate the names of his other nemeses either (Peter the Panda, Planty the Potted Plant, etc.).
Phineas and Ferb are step brothers. Yet in an episode they are trying to help their parents celebrate an anniversary by getting love handel back together. Considering that love handel is meant to be an "80's" band from before the birth of phineas ferb or candace leaves me wondering what the filler should be in this plot hole.
The trivia items below may give away important plot points.
In the series finale, while the kids are riding Phineas & Ferb's grand finale ride, for a brief moment they are transformed into versions of themselves that are animated in the style of a traditional Disney character. Then immediately afterwards into photo realistic versions of the actors voicing them. That is, with the exception of Buford, who is not effected at all and then transformed into an octopus before all of them are returned to normal.
In the episode Rollercoaster: the musical when Candace is showing her mom the rollercoaster you can see the 2 Candaces from the future hiding behind a car. Which means when the future Candace went back to the past she went back to the day of the roller coaster (the Musical) not the first day of summer
In the series finale, Baljeet is looking at a list of possible coordinates on a GPS device. Although the Northern coords are in the correct format, the Western hours are missing. If 00 are substituted for the missing numbers, the majority of the coords on the list would be in Northern Africa.