Hi,
I've done it twice, once with Mauserand9mm's Merkel, which is a factory Merkel DR, not a converted shotgun. I am a lefty and shoot rear trigger 1st, and it was exactly as described above from what we could make out after quite a bit of experimenting to find the reason.
About a month later, my VC 500NE arrived, so off I went to the range. 1st shot, I thought after my previous experience, that I would shoot front trigger first. Same again. This time, the recoil shifted my trigger finger onto the rear trigger and off it went.
Back to shooting rear first on the VC.
It doesn't hurt, but does get your attention!!!
LesLeeSpeed