What do you do to prevent stupid mistakes?

Solution 1:

Surprised no-one has added... go home when you're tired! Brain does not do its best work when you're 10 or 12 hours into the day, go home, grab a beer, get some shut-eye and hit the ground running in the morning!

I also find "peer review" useful... "hey bob, I'm just going to bargle the frargle - you see anything up with that?" just saying it out loud can solidify what you are doing in your mind.

Now we return you to "technical solutions for a single, tired brain" ;)

Solution 2:

I don't do anything to prevent them, but rather, plan around the expectation that I'm bound to make horrific ones.

Solution 3:

Just don't make a mistake that was obviously noted or explained in the documentation... which is a good advice: READ THE DOCUMENTATION FIRST