Bottle too early / too eager, and… BOOM, with beer on the ceiling, and shards of glass from multiple bottles up to 20 feet away. I had to wear protective clothing when disposing of the bottles that were still sealed. I think it was a 5-gallon batch and only 3 bottles had exploded, but the other ~47 bottles were all dangerous gushers.
That’s probably the biggest rookie error I ever committed. The others mostly relate to underpitching. If you want a beer to turn out really crappy, pitch some old dead yeast in there and see what you get.
And then there was the time just a year ago when I first bought my glass carboys. I filled one with hot wort with the intent of chilling it in a cold water bath. Bad mistake. The moment it struck the cold water, it cracked in many places. But that’s not the worst part. The beer was all still in the cracked carboy without any real issue. If I had only taken a hose and siphoned it off into another container, the batch would have been just fine. But no. I decided to try to lift the carboy carefully to pour it into a bucket. Dumbass. It immediately splooshed 100% all over the sink, as of course the bottom circle of the carboy was totally unattached from the cylindrical part.
Maybe that’s actually the worst error, as it was 100% preventable if I had used any brain cells whatsoever, not to mention I had already been brewing for a dozen years and thus should have known better. But I had never ever used glass before that point, so…