The Convair NB-36
If you ever thought that replacing standard fuel engines with a nuclear reactor was a good idea, you need to read up on the Convair NB-36. This plane was so dangerous, it needed supervision from another tailing plane every time it took to the skies, which occurred under fifty times before someone decided that this was all a terrible mistake.
The PZL M-15 Belphegor
Scarily named after one of the Princes of Hell, the Belphegor was the initial biplane every constructed in 1972 as a crop duster. As it turned out, the price of its jet fuel far outweighed any agricultural profits it may have yielded.