We've discussed many causes of bridge collapse, but there are collapses that weren't caused by any of the usual factors -- rather, they were caused by events that can only be described as unusual.
In 1958, Cuba held the second Cuban Grand Prix. Legendary racer Juan Fangio was actually kidnapped by socialist revolutionaries before the race, but that wasn't the worst thing about the event. The course was lined not with guard rails or safety fences, but with spectators standing right at the edge of the track. During the race, driver Armando Garcia Cifuentes lost control of his Ferrari and plowed into the crowd, destroying a temporary pedestrian bridge in the process. Seven people were killed [source: Edmondson].
The Lacey V. Murrow Memorial Bridge in Seattle crosses Lake Washington. It's a floating bridge, suspended on pontoons. In 1990, a bizarre series of construction errors filled the pontoons with water used in resurfacing the bridge along with rain and lake water from a storm. Over the course of several hours, the bridge sank to the bottom of the lake.
The Winkley Bridge was a pedestrian suspension bridge in Arkansas. It was known for swaying significantly under load. In 1989, a group crossing the bridge started intentionally swinging it. They caused the bridge to sway so fiercely that the support structures failed and the bridge collapsed, killing five [source: Bridgehunter].
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Despite what the nursery rhyme says, London Bridge is not falling down and never has. But the bridge spanning the Thames has been rebuilt over and over.