Sure, you could guess that it was sex, drugs or rock and roll that addled Whitesnake vocalist David Coverdale's mind so much that he can't recall with any clarity which music game franchise he recently approved the band's "Here I Go Again" and "Still of the Night" for. And you'd probably be right -- if anyone took too many drugs and rocked a little too hard in the late '80s, it was probably Coverdale.
But don't jump to conclusions. Did you ever consider that maybe he was choosing to make a statement about the genre and its recent creative stagnation? Perhaps by saying, "I can't remember which one," he was actually opining that we haven't seen any real innovation in the "strum on the note" experience since the first Rock Band added other instruments. Perhaps Coverdale was actually calling for both Harmonix and Neversoft to reach out for originality, distinguish themselves and their respective games, and stop releasing copy after copy of what are essentially the same experiences. It was either that, or the too many drugs thing.