I'm new to reloading, only reload 2 calibers, and heres some things I've ran into on my Lee Classic Turret:
Stuck a .223 die in the resizing die when I forgot to swap out the .40 shell holder, and pulled the extractor ring off the case. Took about 10minutes to get it out of the die, but I wont soon forget.
Somehow got a primer sideways in the lee safety prime, and tried to seat it sideways. Ruining two cases in the same week put a tear in my eye, with the way things are.
Buying pulldown projectiles, and not knowing the difference between light pull marks, and heavy ones. Locked my AR up twice before I realized what was going on, and then 3 more times in the same day, before I decided to through all 1000 projectiles and inspect them each one by one. Thankfully nothing bad happened, Just a little embarassing having to tug on the charging handle for all I was worth.