Last year when I was in a plane waiting to take off from Heathrow, the aircraft on the runway (a BA 767 if I remember correctly) powered up for take-off. At that moment a short jet of smoke came through the port engine cowling about half way between the air intake and exhaust. The smoke was still there as the plane disappeared from view.
The aircraft proceeded to take-off without incident and I presume completed its journey safely.
Can anyone explain what may have caused this?