This sounds very like what is known as a "TCAS crossover", of which there have apparently been a few occurences in the States.
It happens when one aircraft is descending towards another which is climbing. Basically the TCAS equipment of each aircraft "sees" the other as head-on and then makes its decision on which way to organise the separation. If it decides that the descending aircraft should pass beneath the climbing aircraft, and both crews follow the TCAS, then both will go through their cleared level, crossover, and then return to their assigned levels. This is much easier to describe with a diagram!
The moral, of course, is to reduce the rate of climb or descent to less than 1000ft/min when within the last 1000ft. And for you controllers out there, it will always help to remind pilots of this if you advise us that there is opposite traffic climbing/descending to 1000ft of our cleared level.
The really frightening time comes when one of the crews decides to ignore the TCAS (probably because they have visual contact). As happened over Japan not too long ago a 747 and DC10 missed by something in the order of 20ft.