Here is a few sample of some of the backflow incidents in Indiana.
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1952
Extensive corrosion of a copper cylinder in a manufacturing plant was caused by faulty check valve on a carbonated beverage machine. Twelve persons ill.

1981
Galvanic corrosion between a gas main and a copper water service caused concurrent leaks, hence a cross connection that resulted in contamination.

1990
Six staff members of a middle school reported becoming ill after drinking water containing ethylene glycol that backflowed from the school's cooling system into the potable water system.

1994
Water main break caused a drop in water pressure allowing anti-freeze from an air conditioning unit to backsiphone into the potable water supply.