updated from WP 3.5.1 to WP 3.6 and the Types (custom fields) plugin (http://wordpress.org/plugins/types/) has started creating empty custom field values even when the associated custom fields are not used.
To be clear, in the 'edit' screen there is a Types section (just below the text editor), no Types fields are added, so in the default 'Custom Fields' section (lower down on the edit screen) there should be no new custom field created, yet for each new post since updating WP to 3.6, there are custom fields created with blank values. Simply deleting the blank custom fields does no good (they reappear). The plugin must be deactivated and the custom fields deleted, but if active, creating a new post will result in blank custom fields again.
Update, I thought it was related to 3.6, and reverting to 3.5.1 seemed to fix things, but nope: Now with whatever version of WP the same issue is happening. I'm trying to find a version history of Types to see if downgrading to an older one might help, but am still looking...