Monday in Sunni, an area north of Iraq, Baghdad AP is reporting a suicide bomb instructor accidentally detonated a car bomb during a lesson on making car bombs and explosive belts. In the explosion, 21 of his own potential suicide bomber students were killed.

From thebusinessinsider.com

"Authorities found seven car bombs — all without plates — several explosive belts and roadside bombs after searching two houses and a garage in the dense area, offering a rare glimpse into the workings of militants behind a resurgence in violence in the country. Bomb experts began working immediately to defuse the explosives.

Security forces rushed to the area after hearing the explosion and arrested 22 men trying to flee the area, including several who had been wounded."

An Iraqi Police Officer says the students were attending a class on how to make car bombs and explosive belts when something went wrong.

I guess it was a live demonstration?