A Briton rang up emergency number 999 to report an UFO hovering above his house, but later said the "mysterious object" was only the moon.
The caller was in a state of panic, saying the object's lights were blazing, The Sun reported.
But a few moments later, he phoned the operator again to say the UFO turned out to be the moon.
Hertfordshire police, which received the call, reminded people not to block emergency lines. The alert was initially treated seriously.
"While the caller may not have been phoning out of malice, his call still tied up valuable police resources and time for something which was not an emergency," said Jason Baxter, assistant manager at the police force's communications room.
"I would like to encourage people before they phone 999 to ask themselves is this really a police matter?"
"If this is not a police matter, please do not call us. Otherwise you could potentially put others with genuine police or emergency calls at risk or prevent them from getting through."
"Also, if you are thinking it would be funny to play a 'trick' this Halloween and make a hoax call, be very careful. You could find the 'trick' is on you when we prosecute you for making a hoax call," he said.
