The Peruvian Puff Pepper - S3-E2
Other mistake: Drake and Josh find the monitor in Megan's room. After they reveal the normal wall and the family leaves, Megan hits the button behind the picture to show the monitors. However, when Drake and Josh find out that Megan entered the salsa competition, they storm into her room, but the button is still revealed in the background.
Other mistake: When Sportacus jumps out of his blimp, his crystal is missing.
Art Class & Lost and Found - S3-E5
Other mistake: Ned paints a rather obscene painting and brings it up to his teacher. He had just painted the painting and his teacher gets angry at the school cutting the art budget, so Ned holds the painting against his chest. When he takes it off his chest, there is no paint on his chest and the painting had no chance to dry before he put it on his chest.