Not so much of a blatant mistake as accident, but a good example of how dangerous beer can be. I was about 2 months into cellar training and starting to do more and more on my own. It was my third time dry-hopping one of the larger vessels (100bbl) and my first time doing it alone. About 80lbs of hops through a 3" port in the top of the tank, maybe 18'-20' high. I get the first half of the hops in before it starts reacting and I cap it up, get off the tank and go about my business while everything settles down which was pretty much SOP for this beer in this tank with the tiny port. This tank had 2 arm, one for the sprayball and another for blow-off, the sprayball arm doubled as the dry hop port so you pulled out the sprayball and capped up the top of the arm and the port in between dry-hops. What I didn't notice was that the arms had been switched during the CRISCO so the whole while I thought the tank was venting, it was really just building up pressure. So after waiting a few hours and not feeling anything coming out of the arm, I get back up on the tank to add the last half of hops. Loosen the clamp slightly to check for any pressure, don't hear anything, go all the way. Bam. 3" stainless steel cap catches me right in the eyebrow followed by a geyser of beer and CO2. I managed to get myself back in the scissors lift and back down on the ground. One of the kitchen guys was walking through the brewery at the time and brought me some ice before people at the upstairs bar noticed the eruption and some of my bosses ran down. One of the scariest things that's happened to me. Worst part was sitting on the floor in a pain and shock-induced daze with my face all swollen up listening to my bosses argue about whether I needed to go to the hospital and insurance crap. Needless to say I left soon after. Learned to always triple check everything, never fully trust my co-workers, and never work for people who don't give a shit about your safety.