In 3.0.6, if I have an item listed in the "Holds to pull", it adds a second item in the list called "undefined". The link for this item is http://10.100.30.53:8080/cgi-bin/koha/catalogue/detail.pl?biblionumber=undefined. This listing disappears when there are no items in the list.
This bug does not exist in master. Since 3.0.x will not be updated again, marking this as resolved.