There were a few issues with some of the first PCIe 2.0 graphic cards working correctly with the original PCIe 1.0 slots. These slots were quickly revised to 1.0a version slots though, and the graphic cards that had problems were also quickly revised.
In short, you would have to have one of the very first PCIe 1.0 motherboards ever made, and one of the very first PCIe 2.0 graphic cards ever made to even set up the circumstances that even maybe, might cause a problem.