OK, let's hear it.
1. Unless it's stainless, a cooler mash tun is not able to be sanitized as an open fermentor (result was my first sour beer)
2. Always remove the blow off tube from the fermentor before you start kegging (result was 4.5 gallons of starsan getting sucked into the fermentor through the blow off tube and ruining a batch of kolsch)
3. Rocks are in malt bags from time to time (got through and auto adjusted my rollers on the mill insuring the crush sucked)
I'm sure I'll think of some more later.
GO!