Factual error: When everyone went up in the hot air balloon the sun and the moon are both visible. The illuminated side of the moon is facing in the wrong/other direction from the sun, which doesn't work.
Tim and Xandir, Sitting in a Tree... - S2-E11
Continuity mistake: When an Entertainment Weekly magazine falls onto the table and Foxxy and Clara both hold it. If you look at Foxxy's hand you see that her furry bracelets suddenly disappear and then reappear.
Tag Team Trial: Part 1 - S1-E10
Continuity mistake: In the opening scene, Crowler is frustratedly walking through the corridors of the academy and walks past several Slifer Red students. In an overview angle, there are 5 students, with two of them standing by a door to the left of Crowler. In the next shot, Crowler is shown walking past the door, but there is no sign of the two students. (00:00:10)