Hnn.. I thought of this one too.. Seeing as I know a lot of guys who have problems with it.

When a girl is having her period, do not tell her she looks awful, tired, sweaty or anything of these likes if you value your dear life. Girls can become extremely cranky during their period, when they yell at you don't take it too personal. But above all, do not ever ask her if she is having her period. In most cases that question will cause a hysterical reaction directed towards you.

Hm... Unfortunately very true. Added it.