At the end of the movie, when David says, guys I've worked a little magic, and produces two bowls of dog food, they are both contain white food, one red, one blue, in the next scene the the blue bowl has dark grayish brown dog food, while the red bowl still contains white dog food.
At the end when David's stepmother is talking to him, she was supposed to say, "Having this baby doesn't mean I don't love you", but instead she accidentally says, "Having this baby doesn't mean I love you".
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 the blue again (purposely).
When Carface is leading both Charlie & Itchifords to Red's hideout they approach a fence, Carface is shown struggling to get through the fence. Meanwhile, Charlie & Itchy have been accidentally layered incorrectly so they are shown as if they were in front of the gate and behind it simultaneously.