If there is a loud hissing noise or the can's contents spew out when you open it, you should probably toss it! These are signs of botulism, a rare but dangerous bacterial strain which attacks the nervous system. Botulism thrives in anaerobic environments (i.e., cans), is heat-resistant, and can be found in "low-acid vegetables." Some vacuum-packed cans may hiss a tiny bit when opening, but when in doubt, throw it out.