Deliberate mistake: When the Trunchbull is drinking from the glass with the newt in it, you can easily see that she doesn't actually drink anything, as the water level stays exactly the same. Logical, as the newt is real, but it's still very obvious because she drinks for about 15 seconds. (00:46:55)
Deliberate mistake: When the Looney Tunes are playing basketball against the aliens, if you look in the stands the same cartoon characters are in different places at the same time. This happens a lot.
Deliberate mistake: After Flipper knocks Sandy off the pier with his tail, there is an underwater close-up of the two of them swimming towards each other. This shot is flipped, note Sandy's watch and bracelets have switched wrists. (00:34:20)