Two planes collided in LAX in 1991 due to what was clearly a controller error, killing 35. Investigators decided that anyone could have made such a mistake given circumstances. The controller wasn't punished, and instead a bunch of changes were made to prevent similar situations in the future.

https://en.wikipedia.org/wiki/Los_Angeles_runway_disaster

Obviously it would be ideal if such changes are made before planes collide, but the US air accident investigation system generally does its job, as far as I've heard.