A man suffered internal burns when he tried to launch a rocket from his bottom on Bonfire Night.
Paramedics found the 22-year-old bleeding, with a Black Cat Thunderbolt Rocket lodged inside him, when they attended the scene in Sunderland.
He suffered a scorched colon and is now recovering in hospital, where his condition is described as stable.
A spokesman for the North East Ambulance Service (NEAS) said the prank could have been fatal.
Douglas McDougal, from the NEAS, said: "We received a call stating there was a male who had a firework in his bottom and it was bleeding.
"He sustained fairly significant injuries in the fact that there's huge damage to that particular area."
Mr McDougal added: "Potentially it could have been a fatal incident.
"There's a lot of major blood vessels round that area, so infection would probably be a huge problem for him.
"And also the body naturally produces methane gas, so combine that with the firework and the exploding effect with methane's flammability - it certainly could have been a lot worse than it really was."
A spokesman for the Firework Association described the bizarre prank as "beyond belief".
He said: "We have spent a long time working with the government to create laws that make fireworks safer and better for the public.
"This incident is very concerning but hopefully an isolated one."
Northumbria Police said they were aware of the incident, which happened in the Dame Dorothy Street area of Monkwearmouth, but are understood not to be carrying out further inquiries.