I once bought the wrong textbook for a class (it was on a related subject, and mis-labeled in the bookstore) and did the first homework assignment out of it.

It seemed to be a pretty tough assignment, which should have tipped me off, as I didn't expect the class to be all that challenging. I did it, though, and turned it in.

The second problem set was assigned, and then I noticed that my book did not have the problem numbers included in the assignment. Someone showed me the correct book, which made me realize what had happened.

After I got the right book, I got the first homework assignment back. I got a 75%.

Needless to say, I figured out then that I wouldn't have to work too much in that class. I do always wonder what the grader was thinking, though - "What the hell is this guy doing? This problem didn't require any simulations..."