The NASDAQ Stock Exchange was totally disabled by a SQUIRREL.

One day in December 1987 a squirrel started a power failure in Trumbull, Connecticut, shutting down the National Association of Securities Dealers' automatic quotation service for 82 minutes. One little squirrel made an impact around the world, halting trading at the Chicago Board Options Exchange, the American Stock Exchange, the Philadelphia Stock Exchange and the Pacific Stock Exchange.

The power failure caused 20 million shares from being traded that day. Another casualty from this incident: the squirrel died.

Oh, the picture? Microsoft CEO Steve Ballmer. But he's looked crazier.
(source