In our company we`ve had a few occasions, involving older Russian aircraft, where following an RA would have caused loss of separation.
Apparently, some older Russian type of altimeters (Gilham-encoded altimeters) give erroneous altitude information to the transponders and therefore the intruder appears to be at an altitude different from the real one.
The cases mentioned occured over China and Mongolia.
Thank god it was VMC at the time.