Both, for me.
Revolver: Border Patrol Ruger Security Six, the trigger return spring broke. Gun would fire, but to cycle for next shot required manually returning the trigger forward. Spring was replaced by armorer and revolver was fully functional.
Ruger Super Blackhawk, similar to Slamfire, the transfer bar wore down where the tip half covered the firing pin. It would not fire if I pulled the trigger slowly, and firing pin impressions were minimal at best. A call to Ruger got me a free replacement and - like Slamfire - I learned how to replace it.
S&W Model 27, primer backed up into the firing pin hole and the gun was jammed but good. Took a rubber mallet to carefully beat the cylinder out of the frame, shearing off that itty-bitty bit of primer. Wiped down and full function restored.
Not directly included are the several times I've had 'gunk' underneath the extractor star. Usually a good blow from my manly lungs will clear it.
Autopistols: Colt Gold Cup in .38 Special. The three leaf (four in the Gold Cup as I recall) somehow bent and was putting pressure on the sear which caused the pistol to go fully automatic. Heck of a surprise on an indoor range. Technically, it would fire, but shooting it was out of the question. Gunsmith fixed with new leaf spring.
H & R Self-loading .32 pistol. Mainspring gave out. Gives nice 'click' but will not ignite primer reliably, very faint primer mark when it does fire. (Other pistols in collection will ignite primers handily.) Have replacement spring, but bent it putting it in, and the pistol won't cock now. Banging head on wall until figure out the rest of it.
Everything made by man will fail at some point or other.