Bungling boffins thought ET was calling home when they detected a mysterious blast of radio signals, only to discover the culprit was their microwave.
Researchers in Australia ended up with alien egg on their faces after they picked up what appeared to be messages from deep space.
They could not understand why the extra-terrestrial activity increased at midday.
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Then they discovered their sensitive detectors were picking up radio signals from the microwave they used to rustle up lunch.
The out-of-this-world gaffe was revealed by David Spergel at the first public meeting of a 16-strong panel of experts assembled by US space agency NASA to examine UFO sightings.
He told the body, made up of experts from scientific fields ranging from physics to astrobiology, of the difficulty in pinning down the source of reported close encounters due to the poor quality of data.
The astrophysicist said it had often been collected with instruments unsuitable for scientific study and was "fragmented" across different agencies.
"It is uncalibrated data, poorly categorised and not well curated,'' he added.
He said the Aussies thought they had made a major breakthrough when they discovered the sudden burst of radio signals.
"They had really strange structure,'' he said.
"People couldn't figure out what was going on. Then they start to notice a lot of them bunched together around lunch time.
"What had happened was that the people in the observatory would heat up their lunch in the microwave. It produced a burst of radio signals that was picked up by sensitive detectors.''
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Mr Spergel said genuine fast radio bursts, or FRBs, had been detected on other occasions.
The "powerful explosions which are taking place at cosmological distances" could produce the "most fascinating objects" in outer space the source of which remain unexplained.
Another phenomenon which could trigger an apparent UFO sighting was sprites — upward-shooting lightning.
He said many events involving UFOs, or Unidentified Aerial Phenomena as the US Government calls them, turn out to be commercial aircraft, drones, weather and research balloons or military equipment.
But he said the current evidence was "insufficient" to provide conclusive evidence about every one due to the lack of quality data.
Mr Spergel called for a stronger approach to data collection but added even then "there is no guarantee that all sightings will be explained."
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Dr Sean Kirkpatrick, director of the US Department of Defence's all-domain anomaly resolution office aka AARO which is probing close encounters by military personnel, said his investigators were now examining 800 — an increase of 150 in a month.
He said "less than single digit percentages" of those cases would stay unexplained.
A recently revealed sighting of a metallic orb captured on camera in the Middle East last year was a mystery, he said, but it may be cleared up with extra data.
"We see these all over the world and we see these making very interesting apparent manoeuvres,'' he said.
But a newly-released video he showed the panel of dots moving across the night sky filmed by a P-3 naval plane in the western US turned out to be commercial aircraft heading to a major airport.
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The panel heard eyewitness accounts of sightings not backed up by other data were unreliable.
One sighting by a US Air Force pilot turned out to be a Bart Simpson balloon.
Astronauts thought they had seen a mystery object in an open Space Shuttle payload bay during a 2008 mission.
It turned out they were looking at the International Space Station 80 miles away.
Former astronaut Scott Kelly told the panel it was common for pilots to experience "optical illusions."
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