The Convair NB-36
At some point in time, someone had the bright idea of replacing a standard fuel engine with a nuclear reactor. In theory, sure, it could work. The fuel vs. nuclear energy debate was high in the sky, but fuel won this round.
The Convair only had around 50 or so flights before someone higher up decided to put an end to it. Even 50 flights is too many considering that it had to be supervised by another plane while in the sky. They're lucky nothing bad happened!