Symptom:
Entity A and Entity B has a 1:N relationship, and on a form for Entity A, placed a subgrid to display Entity B. When you try to add a new record of Entity B in relation to a record of Entity A, the search box displays all records that are related to Entity A, and it displays an error message "This record is already related to other record" when you click one.
In order to avoid this error, user added a filter "Entity A is blank" to the search dialog view. With this filter, search result only displays records that are not related to Entity A; however, now it shows an error "No record found. Please add a new record" at the timing when the user adds a new record.
When the user creates a record CCC for Entity B that is related to Entity A and click save, the title CCC fills in the search box in the subgrid, and a search runs simultaneously, looking for Entity B record titled CCC with "Entity A is Blank," which results in an error (since the new CCC is already related to Entity A).

There is no workaround available for this behavior, thus improvements can be made for better user experience as follows:

1. When the user adds a new record from the subgrid, it will not fill in the search box OR the search box can be hidden.
2. When the user adds a filter to the search box condition, it will not show an error message. (No error when there is no filter set to the search box)