I too have found that if I try to follow a song with one that sounds too similar, I end up singing the first song's melody. Spacing them apart by several songs seems to avoid the problem.
As far as forgetting lyrics or even whole verses, that happens occasionally apart from the above situation. The "unavailable" words usually come back after giving the brain's back burner a little while to bring them up--or if something triggers the memory. Often just getting the first few words is enough to recall the whole thing.