An extremely special 110 kV line opposite direction trip is introduced. The cause for the special trip of zero-sequence current (having direction) is not common protection maloperation and connection error, but a right behavior under the special fault. Moreover, starting from the general phenomenon, this paper gives a thorough analysis of the trip cause by using fault current simulation system according to graph of fault recorder and specification, which not only provides realistic reference for dealing with similar failures, but also presents measures in accord with actual demands for improving the power grid reliability.