Think before you drink!
Beer is fermented grain. It can't go bad. Just like wine can't go bad, it can turn into vinegar but it doesn't go bad.
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facts to back it up from:
http://www.geocities.com/Paris/1265/cvinegar.html
Vinegar is nothing more or less than an alcoholic beverage which has gone sour. In fact, that is exactly what the roots of the word mean, coming from the French vin, meaning wine, and aigre, meaning sour.140 When alcoholic beverages sour it is the act of certain bacteria, known as acetobacter, on alcohol, turning it to acetic acid and water. It is the alcohol gone acid which gives us the taste which we associate with vinegar. It is the other elements, specific to the actual source of the original alcohol, which give the vinegar its individual character and body. The information in the next two sections on the history and making of vinegar is drawn almost exclusively from the pamphlet 'Making Vinegar at Home'.141