An ambulance answering a 999 call was mistakenly sent to a place in Bedfordshire rather than Oxfordshire.
An inquiry has been launched into how the mix-up occurred.
Paramedics were dispatched to aid two teenage girls who were unconscious from alcohol in Grove, Wantage.
An ambulance arrived 50 minutes later, when the pair had gained consciousness. Crews had mistakenly been sent to The Grove, Leighton Buzzard, 50 miles away.
Ian Ferguson, of the South Central Ambulance Service, said an inquiry would investigate the mix-up.
The ambulance service said its log clearly shows a call, on 22 February, was taken from Grove in Leighton Buzzard, but it will check to see if a mistake occurred.
The log also shows a call from Grove near Wantage, 46 minutes after the first call, and an ambulance arrived there four minutes later.
When the ambulance arrived, both girls were up and walking around and able to return home, an ambulance service spokeswoman said.
The South Central Ambulance Service serves Berkshire, Buckinghamshire, Hampshire and Oxfordshire and handles more than400,000 calls a year.
Mr Ferguson said: "This is a very, very rare occurrence. In this case we got it wrong.
"We're conducting an internal inquiry at the moment, to understand exactly what happened.
"It's very important we learn the lessons from this to make sure it doesn't happen again."