The Stirling bridge collapse

On September 11th, 1297, the Stirling Bridge collapsed in the middle of the Battle of Stirling Bridge. There is no definitive account or explanation of the mechanism of the structural failure. Some accounts attribute the collapse to the Scottish saying they deliberately weakened the bridge to sabotage the English attack. 

Some accounts assert that the English destroyed the bridge to hinder the Scottish pursuit of the retreating English soldiers. Some historians say that the bridge was merely overloaded which caused the collapse. In any event, the bridge seemed to have been extremely overloaded and did collapse mid-battle.