When Shrek is eating a bowl full of eyeballs near the beginning of the movie (DVD time index 2.59) he doesn't open his mouth wide enough for the eyeball on the spoon and it passes right through his upper lip. This is only a small technical slip-up in this almost perfect movie.
Well spotted. One would have expected that with the technical advancements in 3D animation that TCD (True Collision Detection-supposed to make an object stop when it hits something instead of traditionally going straight through it) would have become a popular feature by now. Whenever I watch a 3D movie I always watch out for technical goofs like that. If you keep your eyes peeled, you'll find that this happens plenty of times.
Watch a little later when Donkey is sitting on Shrek's chair and gets told to sleep outside...he drops his head childishly and the tip of his hoof goes straight through the cushion. Picky I know, but things like that just bug me.