Doing stuff (like adjusting skimmer or plumbing) to your tanks:
1) after midnight & just before you go to bed
2) just before you leave the house on a trip
3) keeping ick-magnet fish with potentially aggressive tank mates
4) keeping more than one ick-magnet fish in the same tank
and yes
5) starting your first reef tank knowing you are certified as OCD by a doctor friend
(most expensive "mistake" ever)
