Sometimes engineers underestimate the power of resonance and positive feedback phenomena, when a small disturbance is exacerbrated through the feedback loop, resulting in a disaster.A great example of this phenomena is the infamous Millennium Bridge, nicknamed it the “Wobbly Bridge”.
Due to the structure of the bridge, with the cable supports at the side rather than above, many people felt unstable walking across in a traditional manner. In order to balance the natural (initially relatively light) sway of the bridge, walkers widened their walking gait so they were extending their left and right foot movement outwards – giving them greater balance.
As more people began to walk this way it created a huge left right, this cause small sideways oscillations in the bridge due to a positive feedback phenomena.
The bridge started swaying violently left and right!