In the Recess episode "The Legend of Big Kid", at the end when T.J. (as "Big-Kid) has to choose between returning to his friends, or staying with the kindergarteners, his face paint is blue. However, at one part, it turns red! Then, on the next scene, (zoomed out) it has mysteriously turned blue again! I think the colors got mixed up, because T.J's hat is red, too.

(This Slip-Up is also on the Recess film "All Growed Down".)