Do you know what the other aircraft was?

A mode-S transponder reports like any other xponder, but it can report the ALT in 100 ft or 25 ft increments. There are some known problems with ALT sources which provide only 100 ft data...if the mode-S thinks it's getting 25 ft data, it can misreport it's ALT which could cause an TA/RA for another aircraft.

TCAS I provides a TA only.

TCAS II provides a TA and RA.

Also, TCAS II will coordinate it's RA with the other aircraft, but ONLY if both aircraft have TCAS II. If the other aircraft has TCAS I or no TCAS, there will be no coordination...and the other guy might do the wrong thing.

TCAS has given erroneous RA's, due to suspected bad ALT data.

A lot of controllers were concerned when pilots were instructed to follow the RA exclusively, but there are no airliner hull losses due to correctly following an RA. The known hull losses always involved erroneous or confusing ATC inputs...and we almost had another one of those over DEN last year (I know one of the pilots involved in that one).

There have been a few near-misses due to bad RA's but so far they have a better track record than ATC.