Support for this concern...
Just a week ago this actually happened to a fellow Highpower shooter using a Dillon 550. Blew his AR upper into a few pieces...shooter was ok.
Diagnosis was as follows:
Powder bridged in powder funnel and the case was topped with a bullet but no powder.
Next charge fell and dislodged bridged charge, dumping all...the shooter noticed a lot of powder fell all around the case, but ASSUMED THAT THE POWDER WAS FROM A MISSING PRIMER AND CONTINUED TO LOAD. He then put a bullet on the full case (approx 36 grains) of 2230 powder (yes that is a ball powder). And seated it. Sho-nuff, He blew the gun.
What to take away from this:
Assume nothing.
Many of us HP shooters use Dillons, but most of us ream the powder die with a taper reamer to help prevent bridging. I haven't had a bridge yet since I reamed the funnel.
Use a powder that fills the case so an overcharge is obvious. (the charge of 2230 that this shooter uses allows for a large part of a double charge to be contained within the case)
If you have powder dump all over, STOP AND PULL EVERYTHING that's on the shellplate. Something just went wrong. If your powder die just dumped, you have two stations in front of that. The problem could be even further than that, but it will most likely be in the cases that are still in the shellplate.
Correct the condition. My corrective actions (based on this other fellow's misfortune) are to purchase a 650 powder check die and modify my 550 toolhead to accept it (already done by others). Check the powder funnel for rust and re-wax OR have it chrome plated...checking on this). Be more aware of what's going on with the press at all times.
I hope you other progressive shooters get something out of this too.