Dude, We're Gettin' the Band Back Together! (Part 1 & 2) - S1-E22
Continuity mistake: During the song at the end, there is a zoomed out shot of the entire crowd. You can see Phineas and Candace both screaming with their hands in the air, their eyes closed, and Phineas is not holding a camera. When the camera instantly cuts to a close up of them, both of them have their mouths closed, their hands down, their eyes open, and Phineas is holding a camera.
Pooh's Double Trouble / Eeyore Sleeps on It - S1-E19
Factual error: The letters and symbols on the characters' reflections in the mirror look like their regular counterparts, when they're supposed to be in reverse.
The Thrice Cream Man/The Flibber-Flabber Diet - S1-E7
Other mistake: In "The Flibber-Flabber Diet," Chowder and Truffles return when Mung calls them. How could they hear him when they'd already left?
Continuity mistake: Chris says he is handing Heather the final marshmallow of the night. When the shot changes for Heather to yell at the rest of the team, Chris is holding a full plate of marshmallows.
Rocket to the Moon/The Birds - S1-E3
Continuity mistake: "The Birds" - Sheep has the egg in a baby buggy next to the basketball court. She and the buggy are at the side lines. One of the others accidentally hits the ball towards the buggy and Sheep blocks it with her face, scoring a goal. Suddenly she and the baby buggy are in the center of the basketball court.