It's About Time! (Part 1 & 2) - S1-E21
Other mistake: Near the end of the episode after they get back, when the camera zooms in on Phineas and Isabella, Isabella's face is gone - only her eyes remain, but after the camera zooms out her face is back.
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?