Greg Ellifritz over at Active Response Training has a good piece up about the most commonly broken parts on your typical Glock.
It’s good stuff as I can vouch as being an agency and later company armorer for years handling literally hundreds upon hundreds of SIG P-series guns, the most common issues with those were losing sights (get a sight pusher!) split recoil springs (replace every 5,000 rounds regardless), worn out mag springs (rotate, rotate, rotate) and the occasional spun take down lever that just keeps turning but doesn’t engage.
So when Greg talks Glock, listen, as he has seen it.
“I’ve broken almost every Glock I’ve ever owned. No manufacturer is immune from this reality: If you shoot the gun enough, it will break. A gun is a mechanical device and it can fail at any time. I liken it to a car. Even if you buy the best car in the world, eventually it will break down.”