My son put diesel in our gas fired commercial Skag ZTR Lawn Chariot before attempting to mow. He chose a can, the wrong can, not marked, didn't ask, just assumed, and went with it. That didn't play well in the engine. Getting it drained was problematic. After some coughing, sputtering, and smoke it ran OK.
I mistakenly put a flake IMR powder on top of a cylindrical IMR powder left in the powder measure after a session loading 223. I walked away from the 223 session for a day or two and never unloaded the powder measure. At the top of the Progressive Press, the measure "looked" as if it was empty, but there was still powder below the cast rim and below the sight glass.
The 357 Handi-rifle cartridges were pathetic! The first and second to go "down range" went "pifffft", the cast boolit came out of the barrel, and I could SEE THEM going down range about 15 yards in a rainbow arc to the ground. With a baseball glove, I could have caught one (don't try that at home). I had no difficulty opening the action either time, so I loaded a third.
Hmmmmmmm......the brain cells were starting to wake up to the fact that something was amiss. But being the FOOL that I am, I shot the third one. The boolit barely passed the barrel, there was unburned powder from chamber to muzzle, which poured out like water, and was unmistakably two mixed powders (flake and cylindrical). LESSON LEARNED.