Continuity mistake: At the beginning of the movie, Itchy comes to heaven in a teal colored shirt. When given a halo, he is in a blue shirt (the shirt is for heaven, but not given to him at that moment). When Itchy runs to Charlie right after that, his shirt is teal again. When Charlie tells Itchy he is dead, that's when the shirt changes to blue again (purposely).
Continuity mistake: Charlie comes down to Earth from heaven to find Gabrial's Horn, and in the process, wants to go get a collar which allows other people to see him (he would normally be invisible, being an angel and all...)In one scene, before he gets the collar, he is talking to Carface, another dog, and asks him where he got his un-invisibility collar from. The problem is that while Charlie is asking Carface where he got the collar, the usually collar-less Charlie is actually wearing the collar he is asking about. Then in the next scene, he is again, collar-less.
Continuity mistake: When Charlie comes back down to earth for the second time. Near the end and David comes out with two bowls - Sasha's is blue with brown food, but after Charlie and Sasha kiss, Sasha's bowl is pink with white/cream coloured food (just like Charlie's) but with Sasha's name on.