Movie Mistakes? Allright, then. Here's some from my favourite movies:
STAR WARS: The Empire Strikes Back
This is the part of the film when the AT-AT walker collapses. If you watch the back left leg of the AT-AT's model carefully, you can see that it was pushed by a stick (broom, maybe).
This is the part of the film when Leia, Lando, Chewbacca, C-3PO and R2-D2 is trying to save Han Solo. Well, Wookiees don't wear shoes originally...
STAR WARS: Return Of The Jedi
And lightsabers don't have shadows either...