In February 1950 a Convair B-36B bomber jettisoned the nuclear weapon it was carrying when the plane’s engines caught on fire. This was the first lost nuke incident in history.
The bomber was engaged in a practice run at the time which was to include a simulated attack upon San Francisco.
The plane was abandoned in flight by the crew and the bomb was ejected by the crew and detonated in the air, resulting in a large conventional explosion. Five of the 17 crew members died in the accident. The plane crashed in Northern British Columbia.
Here’s a 45 minute video which explains the incident in detail.