This is a somewhat recurring issue. When editing a circulation rule where the "current checkouts allowed" have been left empty, the string "Unlimited" is copied into the input field and has to be deleted. The input should remain empty and the Unlimited should only show in the table for saved rules.
"Holds per record" does the same thing. All other rules seem to behave themselves when blank/undef/unlimited.
It would probably not be hard to fix, but we need to get some attention here! :)