Clicking custom submit/button form element causes error in multiselect validation

I was seeing this error when a "button" form element on a form containing a multigroup was pressed. In my case "Rebuild Table" from the tablefield module was rebuilding the form and refreshing the page to change a table size. This obviously also causes validation as well.

When a form is submitted to be saved $form_state['submitted'] is set to true and _content_multigroup_node_form_after_build runs a few functions to clean up the $form_state array. The same after build function runs in the example above, but because the $form_state['submitted'] is false in that case, the validation function gets passed a malformed $form_state array.

Changing..
if ($form_state['submitted'] && !$form_state['multigroup_add_more']) {

to

if (($form_state['submitted'] || $form_state['clicked_button']) && !$form_state['multigroup_add_more']) {

in _content_multigroup_node_form_after_build should cover this case.

Cross-referencing a similar issue: #730154: Multigroup fields no longer saving; error in content_multigroup.node_form.inc on line 369.
Not sure if these are duplicates. It could be a separate issue.

To test this you can form alter a form with a multigroup on it and add a "button" form element. Click the button and see if this error occurs. Apply the patch and try clicking the button again. Or you could test with the tablefield module.

Better title