In old databases you may have reserves that are attached to a biblio that has been deleted due to a bug in older Koha versions. The circulation screen will not load reserves if the borrower has one of these reserves. I'm not sure what to suggest we do with these invalid reserves, but they do muck up the circulation screen. The work-around for this problem is to chase up the reserve in the database, and remove it manually.
This looks like a data issue - maybe we could add a check to Jonathan's script checking for data inconsistencies? I would assume such a hold could cause problems in other places as well.