Bela P. Havasreti wrote:

Of course, this mid-air could also have been avoided if at least
one pilot had been getting advisories. But always remember
that there are plenty of mid-air collisions on record where both
aircraft were in contact with ATC.

The classic example:

http://www.super70s.com/Super70s/Tech/Aviation/Disasters/78-09-25(PSA).asp