Sway

When it was opened it quickly became apparent that the large number of pedestrians crossing London's Millennium Bridge were causing it to sway from side to side (pedestrians crossing a bridge have an unconscious tendency to match their footsteps to the sway, thereby exacerbating the small sideways oscillations of the bridge). The extent of this had not been anticipated by the designers and the bridge was closed until dampers were added to nullify the sway effect. And this, in my opinion, took all the fun out of crossing the bridge :)

Done