The evidence to me is all the times it's been purported to happen.
Sure, this guy or that guy actually screwed up and double charged a case. No doubt, and SEE gets blamed. I'm sure that happens.
I have a hard time believing that ALL the cases of it happening, frequently enough that the likely scenarios causing it, small charges of fast powders in large cases, has been identified.
What if we say, 80, even 90%, of all instances are actually just stupid screw-up, double, triple charges... that leaves 10, 15, 20% unexplained.
We know we can duplicate the effect, just plug the case solid and load that much powder. BOOM! Now we just have to know why it happens when it "shouldn't".
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.