While we're on the topic of prime numbers, one of the professors proctoring an exam I was taking recently told me that every number that can be divisible by itself and 1 is a prime number. Unfortunately, I think he forgot to include "only" in that definition, so he ended up saying that every number in existence is a prime number.
Was really strange to hear that from someone who has 3 PhD's in math and computer related fields and has had more than twice the number of years of higher education than I've had.
I didn't factor in that he might be drunk, but I have a feeling he wasn't