When merging 2 constituents, you might encounter the error, "Data could not be saved. The UPDATE statement conflicted with the CHECK constraint "CK_GROUPMEMBER_HOUSEHOLDCOUNT". The conflict occurred in database "[DB NAME]", table "dbo.GROUPMEMBER".
This is caused by a conversion issue. The specific problem is that there is an overlapping date range in the GROUPMEMBERDATERANGE table. First try deleting entries from the Previous group membership in the constituent's relationship tab. If that doesn't work, change one of the date ranges from the aforementioned table to remove the overlap or delete one of the relationships entirely via script.