Where there are Lookups using Related Records Filtering, if the show recent records option is also ticked, then the recent records show and allow the user to select recent records that have been deleted - When this occurs and the user saves the record a error message is displayed but it contains no information to indicate why the error is occurring or in what attribute on the form (i.e. it should say something like you selected a deleted record in lookup xyz ).

Similarly, with the same Related Records settings, the user can select recent records that are not part of the related records filter.