A mirror example is in the office_terrain level from the levels archive. There were also some examples in AUM issues.

Also, regard the mirror restrictions: No entities below the mirror plane, not even partially. This excludes mirrors on MDL entities, and also mirrors on transparent sliding doors.

Version 6.50 however will support mirrors through hardware clip planes - then the above restriction does not apply anymore. Still, clip planes are not supported by all 3D cards.