Re: the gun you have on you
Certainly I can think of times when the charge in the cartridge is too weak. This most often happens with handloaded ammo for target shooting. It's why, for instance, one might put weaker target springs into their 1911 if its used primarily for match purposes. I've had this happen several times when I've used match ammo that I've handloaded in my carry 1911.
Incidentally, another point of failure is when the rim of the cartridge case separates upon extraction, leaving most of the case in the chamber. The next round can't be chambered. This often happens with reloaded ammunition where the case has been weakened from excessive cycles of being fired and reloaded; I've had this happen a couple of times with my 1911. It also happens with a number of semi-auto or auto rifles where the factory round is too "hot" and the action cycles before the case has fully contracted after firing. The cartridge case head is ripped off, rendering the weapon useless until the rest of the case is removed. Some older post-WWII rifles came with case extractor tools for just this issue, though I don't think it's much of a problem with modern battle rifles and modern factory ammunition.