It's quite worrying when one has a good number of light aircraft (and some larger aircraft) transponders with inaccurate altitude reporting. One morning the north part of the LTMA went into chaos with a squawking aircraft going through the middle at (apparently) FL70. We later learned that it was a small aircraft at around 1500 ft. TCAS is incapable of knowing that an indicated mode C altitude is wrong, with interesting results.
Despite what the book says about "ignoring" unknown aircraft in Class A airspace, it's sometimes very difficult to do so!