A Devon pensioner's digital television box started a sea search operation when it sent out a distress signal.
The emergency signal was picked up in Scotland at RAF Kinloss. The base alerted coastguards at Falmouth who sent out two lifeboat rescue teams.
Mary Donaldson, 67, from Wembury near Plymouth, said she had no idea her Freeview box sent the signal until her home was traced by Ofcom.
A similar incident in Portsmouth in January also started a sea search.
Mrs Donaldson was confronted by two officials at her front door on 24 January after Falmouth Coastguard had sent out two lifeboat rescue teams.
A spokesman for RAF Kinloss said: "We received the 1215 signal and posted it to Falmouth - they launched and tasked the RNLI boat to this lady's house."
It is not known how the boxes sent out the signal, but both incidents are being investigated by broadcasting regulator Ofcom.
An Ofcom spokesman said that after RAF Kinloss reported the mayday signals, detector equipment was used to trace the boxes. They were then taken away for testing.
He added that both cases were rare instances.
He said: "Any box that is capable of receiving is capable of transmitting, and they somehow flipped from receiving to sending.
"These are two instances in the millions of boxes being used. It's a very small number, and we don't believe it is a widespread problem."
The results of the investigation are expected back within two weeks.
A Freeview spokesman said it was believed the fault was with the boxes manufacturers and the organisation would wait for the Ofcom results before putting together any contingency plan to recall Freeview boxes.
The spokesman said: "Freeview is just the service, anything to do with the signal would be the manufacturer's territory.
"It is not a big issue for us at the moment - it is two boxes out of 10 million we have in the country.
"Therefore we are not putting together a contingency plan yet."