This is such an embarrassing story, but I suppose that it must be told:

I took a multiple choice test once and got something like 40% of the questions right. This was actually one of the better scores in the class, so I did end up making an "A". I should also point out that it wasn't simply multiple choice, but a "matching" test so once you identified that the answer "A" was used in a group, you knew it wouldn't be the answer to any of the other questions in the same group.

To get a 40% on such a test was bad enough, but then the teacher gave us all the answers so we see what we'd done wrong. After sitting back and looking at the test, it suddenly because obvious that there was a pattern:

1=A
2=B
3=C
4=D
5=E
6=F
7=A
8=B
...

This pattern kept up through the whole test (until the last two questions on which he reversed E and F with an obvious question of "The human body is made up of mostly . . ."). I was so steamed I didn't see the pattern (nor did anyone else), but I suppose none of us answered enough correct questions to figure it out. Not a great testimony to the professor's teaching abilities anyway.