I've broken the trigger return spring on my G19. (Replaced)
My brother had a very odd wearing recoil spring assembly on his G17. The spring had nearly eaten half way through the plastic guide rod. (Replaced w/ new assy)
I dropped my G21 and bent the slide stop, it broke when I tried to straighten it out.
(Replaced)
My locking block busted on my G22. It's the steel piece that the trigger & locking pin goes through. The front leg broke off. (Replaced)
My dad broke one of the pins on his G23. Can't recall which one it was either trigger pin or the locking block pin. (Replaced)
My dad has replaced his magazine springs a couple of different times on his G23. (Replaced)
From overly zealous cleaning sessions, I've fudged the plastic slide cover prying it off from the back of the slide. While it hasn't functionally altered my Glock, it does nag me.
If a person doesn't have metal sights, might want an extra couple sets of plastic ones for the odd drop or if your front sight strikes an overhead obstacle during recoil.
If I were super worried about parts damage, my Glock first aid kit would include:
A complete set of pins. (Trigger housing pin, trigger pin & recoil lug pin)
Trigger return spring.
Magazine springs.
Replacement sights. (If outfitted w/ plastic type)
Recoil spring assembly.
Complete slide stop w/ spring. (The spring is easily bent if incorrectly assembled)
Locking block.
Slide cover plate.
Possibly a spare extractor they're cheap & that'd be one spot for routine wear to show up readily IMHO.
That'd about do it. I find it reassuring that Glock replacement parts are "drop in" ready, dirt cheap and take nothing more than simple tools w/ minimal time to replace.
Sly