Suicidal Lemmings
It is a widely held belief that lemmings, from time-to-time, experience a suicidal urge that pushes these tiny creatures to throw themselves off cliffs or into the ocean where they will drown. In reality, lemmings sometimes experience explosions of populations, which result in mass migrations through tight spaces which results in some accidentally being forced off cliffs or to fall into water. So why is it such a widely held belief? You can thank the magic of Disney. While filming "White Wilderness" in 1958, Disney could not film any of these suicides in nature (because they don't actually occur), so they simply forced a bunch of lemmings off a cliff and filmed it.
Source: Snopes.com