Bade 152
The Dresden 152 was not a real aircraft, despite the fact that it possessed strong bomber characteristics. That is because two of the flights in the test series were involved in accidents that resulted in the deaths of the whole crew. East Germany had never produced any other aircraft in its entire history.
Rockwell XFV-12
Creating the first type of aircraft must have been a challenging endeavor from a design standpoint. In the 1970s, you would have believed that the most reputable airlines would not engage in ridiculous behavior like this. But you would be mistaken. Despite its one-of-a-kind design, the Rockwell XFV-12 was never successful in gaining popularity.