I can see it happening, but like most here, I take precautions to prevent it from happening. But mistakes are made, because we are not perfect. I also only allow the one container I am loading with on my bench, all of the others are behind a closed door in a cabinet.

However, using the progressive press adds little extra "challenge" to not screwing things up. The powder hopper/dispenser needs to be emptied as well. I have gone through the motions of changing to a different caliber and forgetting to empty the hopper (when I should, while the previous container is still out). I always start a new caliber by searching through my manuals for a load, then once a powder and bullet is chosen I get them (and the dies) out and put them on the bench for the change over.

I have gotten to the point of changing tool heads (Dillon 550B), only to find out I had left powder in the dispenser. I learned long ago to keep a roll of masking tape nearby and label everything that I'm using. So my hopper still has the name of the previous powder and charge on it until I change it out.

I can see getting in a hurry and leaving a small amount of the previous powder in there, and dumping something new on top. Thankfully I haven't done that yet. Thanks to the OP (kd5exp) I will be even more careful making the switch over.

Thanks kd5exp for owning up to your mistake, sharing them with others (no matter how embarrassing) can prevent a similar mistake from happening to someone else. Glad it wasn't any worse, and I'm sorry for the loss/damage to your firearm.