Like everyone, I forgot a bunch of forms: around 8 various long fist sets, 2 monkey forms, one crane form, two versions of the Yang 108 movement form, a taiji spear set and 3 wing chun forms. I agree with Darth's analogy, forms are like songs, or like dances. If you memorize, say, a waltz choreography and you understand the stepping, timing, structure and musicality of waltzing, then it's really no big deal to forget the choreography afterwards. Unless you have an emotional reason to want to remember it, say, your great grandfather created the routine and it's been passed down in the family for 150 years because he once won a dance contest with it.