What they designed for is one thing, what they achieved is not necessarily the same, and original Colts are now all more than a century old, so are not always in shooting condition. Modern replicas are not Colts. Fit and finish of the Italian clones are not quite up to the same standards Colt had back in the day... though the modern steels are better. In point of fact, even #10 caps fall off sometimes... most often on smaller frame revolvers. It's generally called "cap sucking".
What happens is the cap will expand into the locking notch (not sure of the name) and when you recock the revolver, the hammer will pull the cap off. There are various ways to fix this, the most common way being to plug the notch with JB Weld and carry the gun with the hammer down on an empty chamber.
This problem is even worse if you use #11 caps on a revolver. Number 11 caps do not produce significantly more explosive force than number 10 caps do, they are just a bit larger in diameter. Because of that, they fit looser on the nipples and can allow both a flash to exit the rear of the chamber and for the possibility of said flash detonating the cylinder on either side of the one under the hammer. This is called a "chain fire" and, though the revolver is designed to survive such an event, it is still not something one would choose to experience if one could avoid it.
Also, because of the larger diameter and looser fit, they WILL fall off the nipple under recoil even before they are struck.
Trust me on this, I have had it happen to me when I could not get #10 caps and substituted #11. You need to use #10 caps on all revolvers, some single-shot pistols and even some rifles, though you can cheat a bit on single shot pistols and rifles.
To further compound this issue is the fact that the people who make the nipples and the people who make the caps do not always agree on the proper sizes, so some brands of 10 caps are still too big (or too small) to fit the nipples they are supposed to be used with. When you find a brand that works on your gun, keep using it if you can.
You generally do not have misfiring problems with cap & ball revolvers because they are essentially inline guns, but you can certainly have jams.