It is not possible to delete the delay field in the overdue triggers policy if it is the last notice tab to be edited. To reproduce: Go to an overdue trigger policy (cgi-bin/koha/tools/overduerules.pl) that contains a delay in the 1st, 2nd and 3rd tabs. Delete all of the values from the delay fields in the first and second tabs. Save changes. Delete the values from the delay fields in the third overdue tab. Save. The changes will not be saved. It does not matter in which order the tabs are edited. If the check boxes are also edited at the same time as the last delay fields are edited, these changes will not be saved either, although they can be saved if the delay fields are not edited.
This is still valid (and strange).
Linking bug 25049 - it appears we need some validation for delay changes?