DECLARE @a sql_variant;
SET @a = ROUND(0.5,0);

-- no error
SELECT CAST(@a AS decimal(12,2));
-- error when running on SSMS
SELECT @a;
go

-- Or, the more direct approach
SELECT CAST(ROUND(0.5,0) AS decimal(12,2));
go

.Net SqlClient Data Provider: Msg 8115, Level 16, State 2, Line 1
Arithmetic overflow error converting expression to data type numeric.

BEGIN   TRY
        SELECT  ROUND(0.5, 0) AS a INTO #a;
END     TRY
BEGIN   CATCH
        SELECT  message = ERROR_MESSAGE(), 
                number = ERROR_NUMBER(), 
                line = ERROR_LINE(), 
                severity = ERROR_SEVERITY(), 
                state = ERROR_STATE();
END     CATCH;
DROP TABLE #a;