I have been told that eating food from a can that has been damaged (e.g. from falling) is dangerous, even if the damage is just a dent. Nobody is able to explain me why though so I am not convinced. My reasoning would be that as long as no air can get inside the can, the food is safe.
NEVER USE food from containers that show possible "botulism" warnings: leaking, bulging, or badly dented cans; cracked jars or jars with loose or bulging lids; canned food with a foul odor; or any container that spurts liquid when opening. DON'T TASTE SUCH FOOD! Even a minuscule amount of botulinum toxin can be deadly.
However, dents by themselves are not necessarily dangerous. While they can make the can more susceptible to rust, which can allow in air, which can then result in bacterial contamination, so long as the can is still sealed a recent dent is not likely to be a problem.
The point about sealed but distorted cans is that an anaerobic bacterium (such as Clostridium botulinum) may have produced gases inside the can, and it may have been the pressure of these gases which have caused the distortion to the can.
So the problem here is not the dent allowing contamination (you should in any case avoid cans where the seal has been broken) but the possibility that the contamination caused dents.
Hence the advice in this U.S. Department of Agriculture endorsed leaflet to check for bloating and bulges, to test cans to see if they "pop" back after applying pressure to the dent, and most importantly to avoid those which spray or explode on opening. In other cases, it says "eating from dented cans is not unsafe".