Most forms of foodborne illness can make you pretty miserable, but a few -- such as botulism -- are especially dangerous. The bacteria responsible for the illness, Clostridium botulinum, doesn't make you sick directly. Instead it produces a durable, heat-resistant toxin that's lethal unless treated. Commercially canned foods are usually safe from botulism, but bulging or badly dented cans are an exception.
Crash & Dent Sale
Most dented cans are still perfectly safe, but this depends heavily on where the can is dented and how deeply. The U.S. Department of Agriculture's Food Safety and Inspection Service suggests laying your finger horizontally into the dent. If it's deep enough to conceal most of your finger, the contents of the can probably shouldn't be used. Neither should cans with a sharp dent along a seam, either at the can's side or at the top or bottom corner. Those dents can potentially make microscopic perforations in the can's seal, enabling bacteria to enter and render the food unsafe.
Better Safe Than Sorry
Botulin toxin is a subtle threat because your canned food will typically still look and taste fine, even when it's ridden with the bacteria. On the whole, it's best to simply discard any dubious cans.