There was an accident a while ago (2002) involving a DHL 757 and a Tu-154. I won't go into too much detail here but basically ATC made an error handling these planes and they were brought to close together. They both had TCAS alerts. The 757 followed the TCAS instruction while the 154 followed the ATC intruction and ignored the TCAS which was the opposite instruction to ATC. Both planes collided and killed all board were killed. The ATC controller was later stabbed to death by a family member of one on the victims.

http://en.wikipedia.org/wiki/Bashkir...es_Flight_2937