Staff at the Loro Parque Zoo in Tenerife were taking part in a drill designed to ensure they have an emergency plan in place, just in case one of the gorillas escaped its enclosure.
To make the 'escape drill' feel more urgent, a 35-year-old zoo worker donned a gorilla costume and played the part of the escapee.
Unfortunately someone forgot to inform the vet that it was a training exercise and, fearing there was a real gorilla on the loose, he sprung into action.
Grabbing a tranquilliser gun, he fired a shot - designed for a 400lb gorilla - at the man and hit him in the leg.
He suffered a severe allergic reaction to the shot and was rushed to the University Hospital of the Canary Islands, where he remains in a serious condition.
The National Police now have possession of the air gun, the sedative and have spoken to the vet.
The incident, according to sources at La Opinión de Tenerife newspaper, is considered to be "an accident".