Lemmings do not commit “mass suicide” when they migrate. The misconception stems from lemmings swimming across a body of water together in search of new habitat. In such cases, many may drown if the body of water is too wide.

Disney also made the stereotype of cliff jumping lemmings more popular. The 1958 Disney film White Wilderness, which won an Academy Award for Documentary Feature, included staged footage showing lemmings jumping into certain death after faked scenes of mass migration. The lemmings used for White Wilderness did not jump off the cliff, but were in fact launched off the cliff.

Source of above text: Wikipedia entry on lemmings.