When you assume the role of a moderator and correct people when they make mistakes.
"don't double post"
"wrong forum."
"you just necro'ed a 3-year-old-dead thread. good job"
Etc., etc.
The reason why it's against the rules, is because the moderators here have had problems with it before. Both here and at the Guru, if I remember correctly. It was back in the Guru where they first established this non-backseat-modding rule.
wowzers!
people do that all the time, though D: