One of the big ones for me is forgetting to close valves. I usually write "check all valves" on my recipes. The ants go nuts after a brew day. I tied off a hop bag with a long piece of twine one time. It was so long that it caught fire and the whole thing burned up. The hop bag fell in and it was like trying to pull a dead fish out of water, getting that bag back out. Even when I sort out my hops, put them in separate containers, have everything written on the recipe, and use a timer...I still sometimes forget if I put the last batch of hops in, or if I still have to put them in. Now I actually write down the addition times on a small piece of paper and put it in the container with the hops so I don't forget, but sometimes I forget to even do that. I still brew with open-toed shoes (flip-flops) which is pretty stupid, but I don't like taking on/off shoes...I need some crocs. Convincing myself that I'll clean something later is pretty boneheaded, because I never end up doing it and that never turns out well. One time I wanted to cultivate bottle yeast so I used water and white table sugar...I tasted it...that was pretty dumb. That was also the day that I learned what "CO2 Burn" was...not sure what it's officially called, but it sucks. One time I was convinced that I didn't really need to lubricate my keg gasket, and that some starsan would do the trick. I lost a full tank of CO2, where a finger's-worth of Vaseline would have sufficed.