I emptied a half full powder measure in the wrong powder canister.
Been there done that!
I was quick to find out that it doesn't take much to get sidetracked. I set there at the bench one day with my mind on something else and pressed about 10 bullets in primed cases with no powder in them. I had the scales out the powder measure and the trickler full of powder i just never put any in the case and started loading. Felt like a total dummy once i realized what i had done.
Made another mistake one day not at the loading bench but at the shooting bench. One thing i learned and learned quick was to never try to shoot a 30.06 and a 25.06 together on the same bench. I had loads worked up for both guns and was doing some shooting. I was shooting a 5 shot group and had shot 4 shots with the 30.06 and the 4 bullets were nearly touching. I guess i got excited about the good group and was rushing to put the 5th shot in the target and just wasn't paying attention and picked up a 25.06 round and settled in and fired. The shot didn't sound right or feel right and man talk about a flyer. I didn't know what happened but after about 10 seconds i realized what i had done. I picked up the 25.06 and the ammo and took both back in the house and continued on with the 30.06. I guess i didn't mess up anything because it still shoots great. That was several years ago and i have never shot them both together again and never will.