CREATE TABLE TestIdent (col1 int identity, col2 CHAR(1))
insert into TestIdent values('A')
insert into TestIdent values('B')
insert into TestIdent values('C')
insert into TestIdent values('AB')  -- failed to insert 
insert into TestIdent values('D')

SELECT * FROM testIdent
/*
-- Here 4 is missing
col1	col2
1	A
2	B
3	C
5	D
*/
DROP TABLE testIdent



CREATE TABLE TestIdent (col1 int identity, col2 CHAR(1))
DECLARE @input VARCHAR(5) = 'AB'  -- success for single character.. If size exceeds 1 then error, at that time you should rollback the operations(transaction)
BEGIN TRY
insert into TestIdent SELECT @input
SELECT * FROM testIdent
END TRY
BEGIN CATCH
	SELECT @@ERROR
END CATCH
GO
DROP TABLE testIdent