Just a thought, but I've seen at least a couple of guys shooting double stack .40's (4006TSW's) who had installed their followers backwards at some point when reassembling their mags. Interestingly enough, the guns seemed to feed & fire fine, but the slides wouldn't lock back since the slide stop notch in the follower was on the wrong side of the mag (which is why someone called me over to look at the guns).
Just when you think you've seen it all, someone comes along and shows you something new and silly.