I have trouble even figuring out how taking a cap off a nipple could cause it to be fired! I have never had any trouble taking them off, with a knife, or pliers, or anything else. The tightest caps I ever had to deal with were some foreign #10 caps I used on my first gun, and.36 Cal. Navy Replica revolver. Those cheap nipples peened over very quickly, and those tiny caps held on fire, or not, to those darn nipples. Because there is so little room to work in with even a pocket knife blade, getting either live, or spent caps off the nipples could sometimes be difficult. I started using pliers to remove the spent caps, as that seemed to work the best in the tight space.
I have since learned to check new nipples when I buy them, and inspect them carefully during use. I make sure that whatever caps I buy( I learned to buy them 1,000 at a time) the caps would fit the nipples without problems. I have been using stainless steel nipples on my shotgun, and don't have any problems with caps, going on or coming off. I did open the " hair-lip" on the front of both hammers, to insure that the caps split on firing, and that has made it almost unnecessary to take pliers along. I use my pliers now to help other shooters with stuck caps.