I heard from my professor about last week's test. I scored 45/50, and all the points off were in the last problem.
Yeah, I erased stuff on that problem several times (as well as erasing and redoing the entire first page at least twice). Yeah, I wasn't very prepared. The score reflects not my mastery of the material, but rather my almost-mastery of mimicking what I see in examples (since it's an open book test).
I plan on improvement on the next test.