Baked potatoes in foil can be deadly!

If you bake a potato in aluminum foil and you leave it at room temperature, you can create an environment ripe for a bacterium called Clostridium botulinum, which causes the deadly disease known as botulism.

The reason for this is that C. Botulinum can easily contaminate potatoes and other crop that comes in contact with the soil. Usually, cooking kills the spores, but a foil-wrapped potato holds in moisture, and sometimes prevents the temperature from being high enough to kill all of them.

(Source)