A lemming, sometimes used interchangeably with a sheep, is a term referring to a mouse that follows or mimics the behavior of other mice, often leading to death. The term lemming is derived from the myth that lemmings, hamster-like rodents, commit mass suicide by jumping off of cliffs.
Lemming mice will attempt dangerous jumps and walljumps, often unsuccessfully and regardless of their ability, after seeing it done by another mouse.
The combined weight of lemming mice moving together often causes the fall of bridges and the overthrow of boxes. This can lead to the death or entrapment of mice, due to things like the Shaman jumping, and will quite often be followed by annoyed, "Way to go moron", or bemused comments, "Really, I saw that one coming a mile away." Often newer mice act as lemmings; it is rare that an experienced player will act as a lemming without a plan, such as landing on a post, and often times will make use of lemming activity, such as waiting for them to fall before trying something.
Other times lemmings can cluster together and prevent other mice from moving into the cheese or the hole.
Some mice may troll in a lemming fashion, particularly common in the moving ice block level, where fallen mice may group together to tip the iceberg and prevent mice from reaching the hole after seeing someone else doing so.
In other cases, a lemming mouse may be occasionally useful since they converge in such a predictable manner that a Shaman may be able to use the expected combined force as a weight. Or, a single helpful mouse may inspire other mice to sacrifice themselves for their fellows.