One of the things I've seen cause trouble is the lack of basic weapons maintenance. You trust these tools with your life, why would you let them get filled up with crud that might cause you to lose your life?
One time, when out on a scouting mission, I came across a zombie kill. The guy had been attacked by what looked to be five or six zombies, depending on how you read the tracks. Anyway, his weapon was still there, and after I pried the fingers from the gun, I saw it was loaded with a full mag, but the insides were so gunked up that the gun up and jammed when he needed it most.
You don't have to do anything serious, just run a patch down the barrel and wipe out the insides if you don't have time for a field strip. But at some point you will have to clean the thing, or it will fail you.