Once in Halo 3 we were looking to find the skulls to get the achievements on the level "Cortana". I was round a friends house at the time. We didn't have a clue where it was so my friend went downstairs to look for the solution on the computer and while he was gone I started exploring. I stumbled across the skull before he came back so when he came back he just saw me sitting holding the skull he had been searching for.
I felt very clever.