Most people take for granted the simple, yet effective firing mechanism of the Glock pistols. And to his credit, Gaston did design and build an incredibly robust system that is one of the few portions of the Generation 5 handguns that did not see significant changes. The cruciform, trigger, and disconnector, combined with the slide cuts is a safe and reliable system.
What Glock did not account for was the now plethora of aftermarket options that attempt to improve upon the stock system. This is typically done through the heavy polishing of the various engagement surfaces (which unlike say an AR-15 trigger, there are more than one engagement surface) and to the detriment of some users, changes in the engagement geometries.
The latter can be quite dangerous if the manufacturer did not account for manufacturing variation and tolerance stack. In my case, I can speak to having a Glock go full-auto on me during a 3-gun match. The primary safety issue is the engagement between the firing pin and the sear ledge of the cruciform after reset. Often, especially when using different companies’ parts, the overlap of the two can be quite small.
Shooters not knowledgeable of the platform like this, as the smaller engagement comes across as a shorter trigger pull. That short pull comes at a potentially significant cost for safety. With highly polished surfaces and not enough engagement, its easily possible for the strike to release on reset, not on the trigger pull.
As such, anyone using aftermarket trigger parts for the Glock (or curious to see how their handgun works, should look to pick up an Inspection Slide Plate Cover, which is half an end plate designed so a shooter can see into their handgun and check engagement.
The parts in typically inexpensive, running about $3 from major online sources or coming as part of armorer’s kits like one from OC Tactical, which includes an exploded parts diagram mat, parts tray, and tool. A basic kit like the OC Tactical runs for $26.50.