is O.K.
However, the flavor may get funky as the tin and the food (especially highly acidic kinds like tomatoes) both start interacting with air and each other. (source here and here.)
What CAN be very dangerous about CANS is botulism — this only develops in closed cans, because spores formed by a bacteria called Clostridium Botulinum only produce the deadly (one of the deadliest in the world) Botulin toxin in an oxygen-free environment of low acidity. I’m sure you all know not to eat or even taste anything that comes from a can that bulges or explodes or such. What I didn’t know is that even several hours of boiling wouldn’t make the food safe. (from California Poison Control System)
Favorite research side-result tidbit: “Botulin toxin, popularly sold under the brand name Botox®, is an incredibly potent neurotoxin that has found a variety of remarkable uses in modern medicine and biological warfare. …Throughout the 1950s the toxin was used experimentally in the medical cosmetic treatment of politicians. Then actor-turned-politician Ronald Reagan is rumoured to be one of the earliest patients of this microexpression concealing procedure.” (wikipedia)
