The western United States is full of many different species of scincid lizards, commonly known as skinks. These skinks come in all shapes and sizes, and yet they still try to mate with each other. Too bad it's physically impossible.
That's the finding of US Geological Survey researcher Jonathan Richmond, who has spent a truly heroic amount of time researching lizard sex. Richmond and his team have paired together hundreds of different combinations of skink species. They've found that these lizards really, really want to mate with each other, regardless of their particular species, and even huge variations in size don't seem to deter the would-be partners.
These mating attempts are unsuccessful, and we're not talking about genetic incompatibility here. Instead, the problem is the simple mechanics of lizard sex. For two skinks to mate, the male must corkscrew its body around the female. That probably doesn't bear too much thinking about, but here's the point - skinks need to be roughly the same size for that positioning to have any chance of working. Outside a relatively narrow threshold, it becomes impossible for the two to mate together successfully.
What's really interesting about this is that the skinks are otherwise capable of interbreeding. If one were to try some artificial means of reproduction for different-sized skinks, it appears they'd have no problem producing offspring and beginning gene flow between the species. But in this case, it's the physical side of things that makes it impossible, and that's apparently all it takes to ensure that the different skinks remain separate species.