When holds on items available items are not allowed (allowonshelfholds = don't allow), it's impossible to place holds on items that are 'on hold' (awaiting pickup). The item is not checked out yet, but unavailable to the patron and I think holds should be allowed in this case.

To test:
Set allowonshelfholds to don't allow

0) Search for a record with 1 available item
   Opac status: available
   No hold can be placed.
1) Check out item to user A 
   Opac status: checked out
   'Place hold' button is shown and holds can be placed.
2) Place a title level hold for user B
   Opac status: checked out
   'Place hold' button is shown and holds can be placed.  
3) Return item and confirm hold
   Opac status: on hold
   No hold can be placed.