Continuity mistake: After Amanda calls Angelica to tell her that she can't make it to her party, she takes her RSVP card from the "Yes" box to the "No" box. The problem is, the check mark on the card magically changes from being by the yes to being by the no without anyone touching it.
Continuity mistake: Chuckie is with Kimi and Lil in the school dressed up as the guy he calls 'Chongo', to make Nicole like him. He tells them it's no use and his plan won't work. In the shot when Kimi says to him, "Then forget Chuckie for now, here's your chance to be someone new," the ear-ring he is wearing in his left ear has disappeared.
Plot hole: Everyone tries to get money by selling Dil's inventions, which are all obviously home made, but where did they get so many pairs of sunglasses and where did they get them while they were at school? They didn't seem to bring anything with them to carry beltquariums, et. al...
Continuity mistake: In the scene towards the end in which it shows them as the Rugrats again, when the babies nap it shows Tommy lying on and cuddling with Lil and a space between Reptar and Chuckie. However, in the next frame it shows Tommy transported to cuddling with Reptar a few babies away.