Your dumbest reef related move? Here's mine....

[Jakegr]

I've only been in the hobby for 3-4 years and thanks to my ocd relating to tank safety I've only had one minor flood.

I went away for a month on vacation and left my protein skimmer on with a huge reservoir for the skimmate. Well the day before I got home for some reason the skimmer over reacted, the storage container overflowed, and I got about 1-2 gallons of water on the floor. Unfortunately the overflowing skimmer set the ATO off so salinity dropped... I think I got away with that one though as nothing died. If it had happened sooner, I may have lost a lot of livestock too. So now I turn my skimmer off on holiday. Eventually I plan on installing a float valve in my the storage container to automatically shut off my skimmer in case of an overflow. This can really happen any time, even while I'm at work so I think it's good idea even when not on vacation.

And I'm sure everyone has a money related mistake, here's mine:

Bought an Aqua Medic 2 pump doser for $275. Despite being recommended by my LFS, it turns out the thing is a a POS. So I sell it a month later for $150 to someone that likes them. There is essentially no difference between that and flushing $125 down the toilet.