I once had traffic in the Lambourne hold reporting leaving FL90 for FL80. My next transmission was to clear traffic above (although at the opposite end of the hold) to FL90. Totally legal.
A minute later, when my 'scan' returned to the Lambourne hold - two aircraft, both at FL90 chasing eachother around the pattern.
One was running into LAM and the other outbound from LAM due south of the first by about 3.1 miles!
Lucky that!
AI
