"It is remotely possible to get botulism (the disease caused by the harmful bacteria, Clostridium botulinum) from consuming food from a very badly dented can, especially if the dent is on the side seam or the end seam of the can.
"The reason is that a bad dent, especially on the seams, can cause tiny pin holes in the metal of the can. If conditions were right, Clostridium botulinum and/or other harmful bacteria might be able to enter the can through the pin holes. Clostridium botulinum is a bacteria that is found everywhere, but it grows and makes its deadly toxin only when there is no air is present (we call this an anaerobic condition). It is possible that even with pin holes in a can which let in a tiny amount of air, an airless condition could be found in the food in that can. If this happened, Clostridium botulinum that got into the can could grow and make its toxin; the toxin is what causes the neurological disease called botulism. Botulism can be fatal, but usually is not if modern medical treatment is sought quickly.
"We say it is 'remotely possible' to get botulism from food from a severely dented can because all the right conditions have to be present for this to happen, and many times they are not. So, this foodborne illness would not happen very often, but it could happen sometimes; the challenge is that we don't know if the bacteria is present and if the condition are right in the can or not, so we have to assume the worst and discard severely dented cans of food.
"When in doubt, throw it out."