learning about TCAS/ACAS (Traffic Alert and Collision Avoidance System/Airborne Collision Avoidance System) and it is pretty interesting

and then it led me to the Überlingen Mid-Air Collision incident from 2002 and boy howdy is this case of "regulations are written in blood" if I've ever seen one

and by blood I mean 71 people died in this mid-air collision which caused the new rule to be that if you get a Resolution Advisory from your TCAS that conflicts with what air traffic control is telling you to do, you *always* follow the TCAS instruction since
ATC's picture may be lagging behind and the planes' TCAS are talking to each other

[ if you wanna read up on the collision incident - https://en.wikipedia.org/wiki/2002_%C3%9Cberlingen_mid-air_collision ]