I had a friend in my chemistry class who didn't know any of the answers to a test (the answers were supposed to be written in) so instead of even trying it, she simply filled in the blanks with the story about how she lost half of her finger when she was 5. She made a "C." I had to applaud her.
Here are a few that I've found extremely funny over the years..
There's also a whole book about hilarious wrong answers.