I had an disturbing experience in a SIM once.
On a typical, mandatory, and well rehearsed TCAS RA item, we had been climbing against a generated target. TA was issued, we acquired the intruder visually, called ATC and did not manoeuvre based on TA, waiting for the RA.
Which never came. SIM glitch. As we pushed the nose down, clearing the traffic with about 1/3 the desired TCAS miss distance, all three of us shuddered.