The following EXPLAIN EXTENDED statement produces different number of warning in case it is executed as a regular statement or as a prepared statement. In the later case one extra warning is generated.

CREATE TABLE t1 (c int);

CREATE TABLE t2 (d int);

EXPLAIN EXTENDED SELECT (SELECT 1 FROM t2 WHERE d = c) FROM t1;

2 rows in set, 2 warnings (0,020 sec)
^^^^^^^^ 2 Warnings produced

SHOW WARNINGS;

PREPARE stmt FROM "EXPLAIN EXTENDED SELECT (SELECT 1 FROM t2 WHERE d = c) FROM t1";

EXECUTE stmt;

2 rows in set, 3 warnings (0,001 sec)
^^^^^^^^ 3 Warnings produced (1 extra warning)

SHOW WARNINGS;

The same behavior (one extra warning generated in case the statement executed in PS mode) can be observed if the following test case is run:

CREATE TABLE t1 (a INT, b INT);

EXPLAIN EXTENDED SELECT * FROM t1 WHERE a IN (SELECT a FROM t1 AS x1 WHERE (t1.b IS NULL));

PREPARE stmt FROM "EXPLAIN EXTENDED SELECT * FROM t1 WHERE a IN (SELECT a FROM t1 AS x1 WHERE (t1.b IS NULL))";

EXECUTE stmt;