According to a recently released report from an official inquiry, a sequence of events resulting in the diversion of a Jetstar airliner that occurred on December 19, 2005 was caused when a smaller jet strayed near its flightpath.
The collision avoidance system of a Jetstar 717 was alerted when a Strikefighter ground-attack jet converted to use for tourist "joyrides" strayed too close to the plane last December. The smaller jet came within a vertical distance of 76 meters of the 717, prompting the change in course. In response, the larger aircraft maneuvered away from the Strikefighter.
The Jetstar was never in any danger from the encounter, according to an airline spokeman quoted in a news report.
The encounter was apparently caused by a radio failure on the part of the smaller jet.
[Edited to clarify timeframe.]
[Edited 2006-06-28 20:03:58]