For me, it's getting frustrated with what I make. While I like most of the things I create, sometime I make something I feel pretty terrible about. Then I have to decide if I'm going to carry the idea through anyway, start over altogether, or figure out how to change what I made into something I like. But that doesn't happen too often. If I get stuck, I switch to something else for a little bit as a break and go back to it with new eyes and hopefully a new idea. :)
Best of luck in your endevours.