It's easily bent or splayed, making it impossible to re-install.
New ones are frequently "pre-splayed" and can be impossible to install unless you know the trick.
1. get two pair of pliers.
2. grab the loop-end with one pair
3. grab the legs-end with the second pair
4. twist the legs-end until the legs are no longer perfectly opposite each other
5. bend each respective leg inward SLIGHTLY
6. use the pliers to re-align the legs so they are again opposite each other.
The pin will no longer be splayed at the ends of the legs.
The ends of the legs should now come together as a point, and will guide easily into the bolt carrier.
If the ends of the legs have sharp angles, the angles can be reduced so that the ends of the legs don't "dig" into the carrier as the pin is being installed.
Replace the pin every so often, especially on carbine-length guns, as the pins tend to get pretty chewed up with extended use.
There is pretty much no financial reason not to have TWO LIFETIMES supply of replacement cotter pins. Buy a dozen or more, and un-splay them all, and then distribute them in your shooting gear so that you are never far from a ready replacement part.