Mainstream - S3-E3
Continuity mistake: When Kitty goes to get a midnight snack, her hair is down. But a few shots later, her hair is up in a ponytail.
Fear Burns Down to Ashes - S4-E15
Continuity mistake: Rev Bem returns for this episode, however the costume department seems to have forgotten all the changes his character went through in S3 "What Happens to a Rev Deferred?" His face is distinctly more Magog than it should be, and his hands have reverted back into clawed fingers.
Continuity mistake: I know that Animators re-use backgrounds but it is painfully obvious that when Keitaro and Narusegawa are leaving the exam results posting they walk in a straight line past the same "Ristorante" three times.