On most the Glock mags I've seen, the bottom of the slide lock notch in the mag gets rounded. When it gets really rounded, slamming in the mag can cause the baseplate to come partially off, resulting in FTF problems.
I actually had this happen, although it was with an older Glock and mag which didn't have the mag travel ledges found in newer Glocks/mags. That said, I do still see the rounding on newer Glocks/mags as well but I don't know if it can progress that far any more.