A group of metal detectorists were taken to hospital after unknowingly eating cakes apparently spiked with cannabis in a village called High Melton near Doncaster.
Thirteen men and women fell ill at Coil To The Soil's rally on Saturday.
A detectorist spokesman said: "People could have died. It was lucky that no children or people on medication with serious health problems ate the cake."
A 48-year-old woman from County Durham has been arrested and bailed.
Julian Evan-Hart, editor of Treasure Hunting Magazine, said it was a "dreadful incident" but he did not believe it was malevolent.
"I think it was an irresponsible, poorly thought-through joke," he said.
"This isn't a poor reflection on our hobby, the dig organisers or anyone else involved the planning of the event. It's just a case of random lunacy."
A large fleet of medical staff were called to the scene, including eight ambulances, a rapid response vehicle, two doctors, two clinical supervisors and a Hazardous Area Response Team.
Yorkshire Ambulance Service said two people were treated on the spot.
It said: "Yorkshire Ambulance Service received an emergency call just before 18:30 BST on Saturday to reports of a number of people feeling unwell at a national metal detecting event...
"They had become unwell after ingesting an unknown substance."
Mr Evan-Hart said the cakes were homemade, and had not been sold.
Police said the 13 people admitted to hospital, who came from various places across the country, were later discharged.