Many of the SKS sold were military surplus from the various source countries and were usually covered in heavy preservative grease. The grease got into ever nook and cranny (as it was supposed to,) and could prevent the firing pin from moving freely. If you didn't disassemble and clean the bolt and make sure the firing pin could move freely a slam-fire would be possible if the firing pin was stuck sticking out of the bolt face.
The other way to cause a slam-fire is if the hammer catch is worn so the hammer isn't caught and held. What happens then is the hammer follows the bolt as the bolt returns to battery and if a new round was chambered the hammer, pressing on the back of the firing pin and forcing the pin to stick out from the bolt face can cause the round to fire. Either way it's bad because both problems can cause the round to fire before the bolt is completely in battery (fully forward with the lugs or locking mechanism engaged and the round fully supported by the chamber.) That can lead to the rear of the cartridge rupturing and venting burning propellant, usually into the magazine.
Interesting, I had heard about the cosmoline issues, but wasn't familiar with the issue with the hammer catch. I guess that is to be expected from combloc surplus. Thanks for the information.
It's definitely not widespread, but it's also not just an urban legend. It's pretty well understood how to intentionally recreate the conditions for a slamfire.
The slamfires happen when the firing pin sticks forward in the bolt due to some sort of contamination which isn't anything unique to the SKS. It's probably more commonly associated with the SKS just because so many people pick them up covered in cosmoline (preservative wax) and just fail to clean them properly. You could probably also do it by slathering oil all over the firing pin and then repeatedly operating the gun in dusty and dirty conditions without cleaning, but really at that point it's kind of on you. (E.g., no slam fires here despite filling the thing with mud: https://www.youtube.com/watch?v=SPL1gEqBk0s)
Take care of your tools and they'll take care of you. I've put many thousands of rounds through mine without issue, and I know a lot of people that own them because they're extremely cheap here and I've never run into anyone in meatspace that has experienced a slamfire. And even on the internet many people _warn_ about them, but relatively few claim to have ever experienced one considering how common the rifle is. That said, I still make sure mine is pointed downrange every time I chamber the first round from the magazine because it doesn't hurt to be extra cautious.
In all the rounds I've put through mine I've had exactly one failure. An old, corrosive surplus round manufactured in Czechoslovakia in the 1970s. It seems to have expanded a little more vigorously than the usual round or something and got stuck in the chamber. Still fired correctly and the extractor did its job and... ripped a chunk out of the lacquered casing trying to pull it out. Had to pound it out through the muzzle with a brass rod.
They're pretty much what you'd expect out of a soviet russian firearm. Overbuilt, rugged, reliable in hands of an untrained peasant, and somehow no two came off the line the same.