Two light aircraft came within just a few metres of each other as they came down to land after neither of the pilots heard the other's radio calls.
As a Cessna-152 was landing at Clacton Airfield, Essex, on 23 June, its pilot saw a C42 fly about 10ft (3m) above it.
The pilot of the C42 said he did not see any traffic and the other plane may have been under his nose or hidden by the housing estate underneath.
The UK Airprox Board assessed the risk of collision as the highest possible.
Its report said the C42 pilot saw the C152 heading north at about 1,000ft but believed he heard a call that it was leaving the circuit, so he discounted him as traffic.
The C42 then made a shorter circuit to avoid flying low over houses and then tried to land, at no point seeing the C152.
He aborted the landing because he "fortuitously", the board said, failed to lose enough height and speed but heard the C152 pilot ask "where he had come from?".
The air-to-ground frequency at the airfield was not manned at that time and neither heard each other's call on the radio.
"The board could not determine why the two pilots had not heard each other and wondered whether it had been aerial blanking or simply just not assimilating each other's transmissions," the report said.
It also added the near-miss highlighted the need to be vigilant when there was no air-to-ground frequency, as well as the dangers of flying non-standard circuits
The cause, it said, had been the C42 pilot "did not integrate with the C152 ahead in the visual circuit".
It concluded: "There had been a serious risk of collision because neither pilot had seen the other before the Airprox, no avoiding action had been taken and, at the reported 10ft separation, it was clear that providence had played a major part in collision avoidance."