19. BMW M3 E30
We are sure that we would get crucified for this claim, but the legendary E30 M3 is kind of a flop. Yes, it is one of the most successful racing cars ever made, and it is highly sought-after by collectors, but at the end of the day, BMW lost money on them. It was a fantastic competition model but it failed as a road car (via Road and Track).
The reason was the S14 engine, which was powerful for the times (and for being a four-cylinder). But it was too fragile, problematic, and unreliable. It wasn’t as fast as people thought it was and maintenance bills were enormous. If you compare it to its biggest rival, the Mercedes 190 2.3 16V, you can see that BMW failed by only managing to sell 17,000 examples while Mercedes sold three times as much. Learning a lesson due to the E30 M3, BMW changed the philosophy with subsequent M3 models and sales were much better.