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
Revealing mistake: In the beginning of the second half of The Birds, the group is using binoculars to watch a bird flying around in the sky. Frog's movement and line of sight do not match up with the bird's movement. The others seem to do just fine.