Fallacious yet widespread and documented beliefs courtesy of Wikipedia.
Lemmings do not engage in mass suicidal dives off cliffs when migrating. They will, however, occasionally unintentionally fall off cliffs when venturing into unknown territory, with no knowledge of the boundaries of the environment. The misconception is due largely to the Disney film White Wilderness, which shot many of the migration scenes (also staged by using multiple shots of different groups of lemmings) on a large, snow-covered turntable in a studio. Photographers later pushed the lemmings off a cliff. The misconception itself is much older, dating back to at least the late nineteenth century.