After a lot of investigation I found that a problem I was having uploading attachments on a 5.10 site originated from the call by reference on $form_id in this blockregion module.

On file upload I was getting a dialog box with "An error occurred: Unspecified error" - as also reported at http://drupal.org/node/248146. That dialog box is triggered by Drupal.parseJson in drupal.js.

I found that disabling this blockregion module solved the problem, and then on further investigation found that getting rid of the call by reference on $form_id meant the blockregion module could be enabled without the error re-occurring.

Why this should be so, I really do not know.

There seems to be no reason to have $form_id pass by reference, and I suspect it is just a coding error.