I pulled this trick in my entry-level calculus class in college. They were so generous with extra credit that my marks should have meant that I got an A. But I didn't do any of the homework and the TA decided to punish me with an A-. I suppose I should have fought, but I'm epically lazy.

Lesson: if you're going to do something like this, don't assume that the people on the other end will hold up their end of the implied bargain. Either fight or do some token tribute.