Had a few doozies over the years.
On my first brew I had the obligatory put airlock into hole, and push gasket into bucket. This was after 3 days of fermentation and I pulled the airlock out to look through the hole at my beer. Didn't want to reach in with my hand so I bottled right then and there. Good thing the beer must have been done as I never had any bottle bombs. Of course the beer tasted like crap but doesn't everybody's first?
Just finished bottling a case of beer. Put them all in a 24 pack with the caps resting on top, to carry to my table to cap. As expected, the bottom falls out of the case dropping all 24 full bottle of beer onto my kitchen floor. I live in an old house so naturally the floor isn't level, all the liquid pooled into one corner of the kitchen and cascaded down into my basement. That was fun to clean up.
Destroyed my hydrometer a multitude of ways, dropped a bottle of beer on it, roll off table onto concrete floor, sat on it, stick in shirt pocket to bring upstairs after brew day it falls out onto stairs, cat playing with it, drops it onto concrete floor, reaching for something on brew table, knock over oxygen tank which starts a chain reaction resulting in hydrometer dropping on concrete floor, I use a file cabinet to store my brewing additives/equipment, close drawer didn't notice tip of hydrometer sticking out...broke it. Those are what I can remember, lol.
My most expensive mistake: Buddy and I were collaborating on a rum-barrel aged coconut porter. Everything went great, the base porter tasted awesome (I bottled a 6-pack prior to adding oak spiral/rum), it tasted even better after I added the expensive rum that had been aging on the oak spiral for a month.
We decided to use Doc's method of using coconut (from the brewing network) which is to add extract. We decided to pull four 4oz. samples and dose with increasing amounts of extract to see what flavor profile we liked best. Seems fool proof. Well we were drunk when we did this, and of course we chose the strongest mix. Then to top it off, I didn't have quite 5 gallons in the keg which is what we had calculated for, but in our drunken haste we didn't consider that. So basically based on our drunken calculations we need to basically add 3/4 of the bottle of the extract to the keg. We had the bright idea that the coconut would age out, so WTH why not just ad the whole bottle of extract.
There was SO much extract that the beer actually gave us headaches and nausea after drinking just a few ounces. Turns out Doc adds just 2 drops per bottle, so we added 116 mls versus the 7mls top that we should have added.
We tried to salvage it by brewing another 2.5 gallons of the same base porter and blending it with 2.5g of the poison porter, and it was STILL way too over the top coconutty. Needless to say we ended up drain pouring that whole batch after spending a $100+ and God only knows how many hours into the beer. Only saving grace is that I think we at least learned our lesson!