I just received the following error in my lab when i tried to create a new external network.  This seems to be a known issue in vCD 5.1 & 5.5 but has not been resolved in v8.

I did not delete any port groups or VSE’s directly from vCenter server so assume the database was not successfully delete when i removed external networks.

VMware provides a comprehensive KB article on this so not going to get into to much details but solution is to remove the stale entries from Resource_Assignment table in the database.

KB: 2042474

Links:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2042474