2. Lack of prep
Doing maintenance or research on brew day is a bad idea. Accidents are bound to happen. Brew day is complicated enough without adding tasks like maintenance and research.
A – I had a bad thermometer on my boil kettle, so on brew day I told my son to replace it, but later; after I added the hot water to the mash tun. An hour later I said OK, time to replace that thermometer. So he goes out and removes the thermometer screwed into the mash tun, dousing himself in a gallon of hot wort. By the time I ran out to help plug the hole, I don’t know how much we lost, but it was more than I wanted.
B – Do your research ahead of time. When you have 5 minutes until the next hops addition, now is not the time to see if you can replace one type of hops for another without missing your target bitterness/flavor/aroma. Besides, while you’re caught up in research, who’s watching the boil? (See below).
3. Failing to use a checklist
When you’re first starting, you haven’t yet developed brewing habits. For example, I put on my seat-belt automatically when I get in the truck. One time I moved my truck just from one side of the driveway to the other, and then tried to get out only to find that I was strapped in, and hadn’t even known it. Until you develop these automatic behaviors, use checklists. They keep you from forgetting things or missing steps. Go over the checklist the day before, and also use it on brew day. Just do a search for brew day checklist.
4. Boiling/Fermenting in too small a vessel
Whether you’re boiling or fermenting, you need headspace. When the wort heats, it expands. Also, at the beginning of the boil you get hot break, which causes boilovers. I do my 7 gallon boil in a 10 gallon pot, and that’s as tight as I want it. Primary fermentation results in a foamy layer called krausen, and you need head space in the vessel for it.