A conversation on another site reminded me of something fun that happened on one of my exams last fall.
If class attendance gets low enough, I'll walk off camera and/or turn off the mic to give the attending students the answer to a bonus question on the exam. The question is a fill-in-the-blank question with the prompt something like "What did I draw on the board on October 15?"
On one of the exams, about 3x as many people had the correct answer as had actually attended. I realized I wasn't rewarding those who attended so much as rewarding those who had a friend who attended.
My horror at this realization was offset by my amusement at another realization. There was at least one student who refused to provide the correct answer -- there was a sizable number of students (about 2x as many as had attended) who all had the same incorrect answer.
