I've heard extremely well known, long-time performers forget words to songs. I won't mention their names here but everyone would know them. It happens. I've heard a a friend of mine who has recorded countless albums and traveled all over the world singing forget the words to a song while doing a concert set at a large festival to the point where the person had to finally give up and move on to the next song.

Songwriters forgetting words to their own songs is not surprising to me. After all, they've probably changed the words many times during the process of writing it before finally settling on particular words. The old, discarded words are still in there.

Jerry