Well, if you want to include the pistols failing to function when adversely affected by shooter & ammunition related factors, as well as parts wear/breakage/failure ... then I'd have to say that I've seen some problems occur with a number of Glock pistols (including my own) ... just as I have with most other manufacturers pistols over time.
Just the other day a fellow from another agency experienced his G22 extractor breaking, causing repeated failures to extract/eject.
I had a failure to feed with a new G27 magazine the other day. May not have been related, but when I was taking the magazines apart for cleaning I noticed a lot of 'flashing' sticking out (inward) on the inside front & one side from the molding process. The failure to feed could've easily been caused by a momentary slip of my wrist, though, or was just one of those occasional 'sometimes' things that can happen when breaking in new equipment (magazine). The ironic thing is that the new magazine was replacing one which had started to exhibit feeding failures I suspected might've been caused by feed lips which had become slightly out-of-spec.
Seen other broken extractors, broken guide rods, deformed guide rod lips (G26), weakened recoil & magazine springs, cracked channel liners, cracked magazine bodies, slide stop lever springs, 'revised' magazine followers which corrected feeding issues ... the assorted sorts of things you might reasonably expect to sometimes see over enough time and normal service use when you see a reasonable number of pistols used ...
Glocks are fine pistols. I enjoy mine and have confidence in them ... as I do with several other fine pistols made by other manufacturers, as well.
Pistols are machines. Proper use, maintenance, and periodic examination can help maximize their useful service life ... while improper maintenance & modification, abuse and neglect can create the potential for problems.