I've just upgraded to Views-6.x-3.x-dev from Views-6.x-2.16
and I'm now getting this error on my site.
Looking at $arg, which is being passed to html_entity_decode on this line, I see that it has an array with the NID in it, eg:
 => 14687
In one case I'm also seeing what looks to be partial contents of a form being held by $arg
Incidentially, I'm also getting a few of these errors:
warning: html_entity_decode() expects parameter 1 to be string, array given in /home/drupal/sites/all/modules/views/handlers/views_handler_field.inc on line 1070.