The Millennium Bridge, a footbridge spanning the Thames from St. Paul's Cathedral to the Tate Modern gallery, closed two days after opening (on June 12, 2000) due to some unforseen stability issues (it wobbled when people walked on it).
Interestingly, the error in the construction of the bridge wasn't necessarily the fault of the architectural or engineering firms that constructed the bridge. The error wasn't caught because the behavior seen on the bridge wasn't part of the standard testing process at the time. They didn't know to check for it because this particular problem had never come up in the computer simulations. The computer checked for potential vertical vibration caused by foot traffic, but didn't check for potential lateral (side to side) motion due to foot traffic because no one really knew that problem existed. A similar thing happened with the Tacoma Narrows Bridge in Washington State. No one at the time knew to check for possible vibration or twisting due to wind, causing the bridge to be torn apart by 40 mph winds.
Over a year later, they're still making repairs on the Millennium Bridge. I'm sad I didn't get to walk across it; I had to settle for this photo of the bridge with the Tate Modern in the background.
