The original diagram shows cached/posted forms re-entering the flow right before validation. Actually these forms are run through drupal_process_form() -> form_builder() regardless. (Which gives people a crack at things via #after_build before validation :) )

See http://codelica.com/misc/FAPI-diagram-corrected.jpg

[Thanks, and we've tried to get an updated diagram into the second printing. -JV]