I've managed to fail to get the protection back on/in a couple of times.
In a Combat focus shooting class I manages to not get the right plug in all the way when we started a new string of fire. (Naturally I was on the left end of the line) I quickly fixed that problem, but man it hurt.
The only time I generally fail to use at least plugs is hunting, but then again I'm a crappy hunter so I almost never get a chance to shoot while hunting.