Mode error
A mode error is when a user of something makes a move that is appropriate to a different mode. Depending on the design, the results can range all the way from a minor annoyance to a full fledged disaster. The annoyance happens most often in computer software, as described in the Excel external link. An example of a case that could be a disaster is if a driver is conditioned by a four way stop sign near his or her home to assume that similar intersections will also be four way stops. If it happens to be only two way, the driver could proceed through if he or she sees no other cars, and especially if there is an obstructed view, a car could come though and hit the first car broadside. This is why modes should be avoided if at all possible. If modes must be used, there should be clear indicators of the current mode to help prevent mode errors.
See also
Human computer interaction design