Euler's Identity. If you Google the answer, that appears in the names of the first three results, and sometimes appears in the descriptions of others!
Category: History?, Information? (depends what category Urban Legends fall under)
Question: What legendary ghost may be summoned by chanting his/her name?
I always thought that even though Urban Legends can be quite far fetched, they're always interesting.
Whether true or not, this one's one you DON'T want to test.