...that don't taste great together.
When I went to Japan recently, I encountered a fairly fancy gelato store that served some unusual flavor combinations. Even though it was the dead of winter, I had to try their "sweet cream and balasamic vinegar" creation. That's right: sweet cream gelato, with balsamic vinegar on top.
No, that's not chocolate sauce on top. Surprisingly, the offsetting effects of the tangy vinegar and the soothing gelato went very well together. That's an example of an idea that sounds horrible, but wasn't bad in practice.
The following, on the other hand, is probably an idea that initially sounded great, but is actually a horrid, horrid example of marketing gone wrong (though marketing hardly ever goes right).
Sure, combining Transformers with Star Wars might sound like a good idea, if you're seven years old. But, to quote Adam Sandler, who were the ad wizards that came up with that one? Hey George, ease up on that rubber stamp.