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.
Other mistake: The first "All Dogs Go to Heaven" was set in 1939 and Charlie died that year. When Itchy joins his pal in Heaven, it is obviously the 1990s. Are we expected to believe that Itchy lived for over 50 years compared to other dogs? If so, why isn't he extremely old when he finally arrives in Heaven?