A Poisonous Potato is a food item that can be obtained (5% chance) by harvesting a regular potato plant. A poisonous potato can cause rapid damage to the player if eaten (similar to a Spider Eye). However, it is not quite as dangerous, because it only has a 60% chance of poisoning rather than a certain chance. When obtained, the poisonous potato cannot be baked or planted like a normal Potato. These were added in the Pretty Scary Update (1.4.2).
If you get poisoned by a poisonous potato it'll give you "Poison" for 5 seconds.
Eating one of these at night is very risky, as it'll fill the hunger bar but deplete your health. If you come face to face with something like a skeleton or zombie, you will have a higher chance of death than you normally would.
You can tell if a potato is poisonous because normal potatoes do not have green around them.
Another way to tell is that it will not stack with regular potatoes.