Even with something as small as a mousetrap, traps can go bad. You may have had an experience with a common household snap trap in which the animal only got caught by a portion of the trap and didn't die instantly. Such things can also occur with other outdoor traps, causing an animal to suffer. This is one of the reasons trappers are supposed to check their traps often.
And although there are rules and provisions to try and protect against accidental trapping, traps are not intelligent -- sometimes capture animals they're not meant to. They aren't like hunters, who knowingly aim only for a buck or a pheasant. Threatened and endangered animals can become trapped, as well as domesticated animals. In some rural areas, humans have to be careful to avoid large foothold traps intended for bears and other animals. In catch-and-release cages, it may not be as much of an issue if the incorrect animal is trapped, but it can still be extremely stressful and detrimental to a wild animal's health to be locked up in a tiny cage for an extended period of time.
Though trapping can be very useful and very humane, even the best intentions sometimes have unintended negative outcomes -- in this case, animal cruelty. The complex issue of trapping is not likely to be determined anytime soon, so for more information, visit the links on the next page.