Do you ever space out while practicing? Does your mind wander? Could it be that you might be boring yourself by dusting over the surface of the music? Could it be that you are playing out of tune, and you don't want to deal with fixing intonation at the moment, so you stop paying attention? Could it be that there are things that are physically difficult to play, and by not paying attention to them you avoid the problem of facing the fact that playing such passages requires strategy and forethought?
This happens to me from time to time. No. This happens to me often. I'm trying to make it happen less often, and I know I can, because I'm the only person in charge.