Contact merge not displaying custom checkbox field values - displays nothing or 'Array'

I found this bug while preparing to merge 2 duplicate contact records on civicrm.org's production DB.

http://civicrm.org/civicrm/contact/merge?reset=1&cid=2575&oid=2651

Both records have values in the 'Contact type' custom checkbox field. However, when you view the above merge table, the left side is empty (should display the single checkbox value - 'Administrator'), and the right side displays 'Array' (this record has multiple values checked for the custom field).

I've left the records unmerged so you can see the issue recreated easily.