The mag is seated to deep, and hangs on the ejector. I had that happen during a run with my ICE and a CMC 10rd power mag. I have no idea how it happened because I left the stopper where it belongs, but had to dremel down the sides of the pad to fit against the magwell. You go to seat the mag, it pushes into the ejector, the slide wouldnt close and had to pull the mag back out. I'll be testing that some more before I seat one of those hard again.
I have encountered (created) this problem too with my setup (9mm Trojan, Ice, ETM mags). One more reason not to shoot to slide lock!
Totally agree about speed vs. margin for error for any given reload. But over the long run errors slow you down, so more margin of error = fewer errors = less time.