I needed to do some custom validation of fields on a form. So, I decided to use #element_validate. One of the fields I was validating appeared a bit strange to me, though. When I displayed its $form_state['values']['field_face_palm'] information I saw that it looked like:

$field_face_palm['und'] = 'you_knucklehead'

instead of like:

$field_face_palm['und'][0]['value'] = 'you_knucklehead'

read more