Ugh.  This ever happen to anyone?  I've got an 870 express with an extended mag tube, which I keep loaded with 00 buck and slugs.  After leaving it alone for a couple of weeks, I unloaded it for some reason, only to find that one of the slugs has deformed from the pressure of the spring, and will no longer even chamber.  

Has this ever happened to anyone, and is there anything I can do to keep it from happening again?

Presumably it is the shell itself that is deformed, not the lead slug.

I’ve heard of this happening in police shotguns stored in a hot area (say in a cruiser trunk).  The heat weakens the plastic hull, allowing it to collapse a bit under the pressure of the spring.  Long-term storage of a fully loaded shotgun probably contributes to the problem.

Assuming you’re not storing your shotgun in an especially hot area, this may have been just one weakly constructed round.  Or it may be one particular brand of ammo.

However, extended magazine tube springs put a lot of pressure on the rounds when the tube is fully loaded.  If this is a recurring problem, you might want to underload the tube to relieve some of the spring pressure.  (Alternatively, I guess you could keep your shotgun in the refrigerator!)

I won’t ask why you have both slug and buckshot loaded together – it’s your shotgun.