I have mentioned elsewhere that the SPAS-12 is quite complicated to use, at least as far as shotguns go. If you don't practice you'll likely find yourself tugging at something and wondering why it won't budge. This is especially impressive in front of your friends who you were (up till then) showing off to.
There is one primary mechanism for inexplicable jams. To see it in action, do the following:
This is so easy to stumble across. Fire it till it is empty, put the safety on, load it up, and then try to chamber a round. No way.
Or load it up with the chamber empty and it uncocked to save the spring, put the safety on and put it somewhere ready to use. Grab it, pump it, jam! If a round is not chambered, don't put that safety on! This jam especially impresses household invaders.
Practice makes perfect!
If you try to cock the SPAS with the safety on, you damge it as it hits, see below. Cocking it includes trying to open the action. Don't do that!