Working out callback ordering is tricky
There are now lots of levels of callbacks to work with:

1. Old node hooks (hook_load, hook_form, etc)
2. nodeapi hooks – based on $op, they all happen after the corresponding node hook
3. hook_form_alter – replaces old hook_nodeapi:form call
4. Validate and Submit callbacks set up in form with FormAPI – So there are now potentially 3 different places to do validation

Some ordering:

1. _load
• inputs: &$node, but ignore it – nothing useful in it yet, and any changes are rejected.  Pull data from db.
• outputs: object(hash) that will get merged with node.
• goal: move data from db into node
• ? why not just allow you to modifiy $node?
2. _prepare
• inputs: &$node with info from _load.  Pull data from $_POST and $_SESSION
• outputs: changes to $node persist down chain.
• goal: make changes to $node based on non-submit actions
3. _form (does not go to _nodeapi, now use _form_alter)
• inputs: &$node – now fully loaded with $_POST[‘edit’] array (and data from _load and _prepare)
• outputs: returns the bad-ass form array. Can modify $node, but that would be odd(?)
• goal: build the form array
4. _validate
• inputs: $node: filled with all form_values(RO), $form: form array
• outputs:
  changes to $node are lost, and changes to $form are pointless.  Use
  form_set_value( $form[‘id’], new_value ) to modify the values in $node
  before submission (weird). Use form_set_error( ‘id’, ‘message’ ) to
  abort the submission.
• goal: change to reject the form submission.
• ? why make it so hard to modify $node, but still possible ?
5. _submit
• inputs: &$node with all the trimmings
• outputs: modificatons to $node persist
• goal: last chance for tweaking before heading to the db(?) Mostly for _form_alter chaining
6. _update/insert
• inputs: $node (RO)
• outputs: updated db tables
• goal:
  persist extra node info to the db.  Called after node_u/i.  If this is
  an insert, this is the first moment you have a nid (important if you
  are doing bonus db changes that need to link back to this node).