Zinc poisoning is caused by the accidental or deliberate ingestion of zinc compounds. Zinc is an essential nutrient and is commonly found in nutritional quantities in most grains and legumes.
However, zinc is also used in pennies (which are 97% zinc) and in dental adhesive. Zinc is absorbed readily into the body and dissolves quickly in stomach acid. Zinc interferes with the body's ability to use both iron and copper. Low doses can cause intoxication like symptoms, where higher doses can damage the liver and kidneys.
Luckily, the fatal dose for humans is in the range of 1kg of pennies, so deaths are possible, although rare. Zinc is much more toxic to smaller animals and even a swallowed penny can cause serious health problems in dogs.