use this script for test:
DECLARE @FILENAME AS NVARCHAR(500),
@FILENAMEPRE AS NVARCHAR(500),
@FILENAMESUF AS NVARCHAR(500),
@SQLSTMT AS NVARCHAR(MAX)

    SET @SQL_STMT = 
'SELECT 
        t1.*,
        CASE WHEN c1 IS NULL
             THEN ''Y''
             ELSE ''N''
        END AS [COLUMN1],
        t2.c3
   FROM t1
   LEFT JOIN (SELECT *
                FROM (VALUES
(''v1'',''v1'',''v1''),
(''v2','v2','v2''),
(''v3'',''v3'',''v3'')) RET (c1, c2, c3)) t2 ON t1.c1 = t2.c2 '

if you press "Display Estimated Execution Plan", then the first line will be highlighted with an error: Incorrect syntax near ','.
But if you press "Execute" then correct one will be highlighted