Page Chapter 10. The Form API, Page 172

Location on the page: 
#pre_render examples
Error: 

Incorrect function signatures for foo($form_id, $form) and bar($form_id, $form)

Correction: 

The signatures should have references to the $form variable:
foo($form_id, &$form) and
bar($form_id, &$form)

Description of the Error: 

Missing reference markers.