Comma is not allowed in SET values in a field definition:

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a SET('a','b','c','a,b'));

ERROR 1367 (22007): Illegal set 'a,b' value found during parsing

SP variables erroneously allow commas:

DROP PROCEDURE IF EXISTS p1;

DELIMITER $$

CREATE PROCEDURE p1()

BEGIN

  DECLARE a SET('a','b','c','a,b');

  SET a='a,b';

  SELECT a, a+0;

END;

$$

DELIMITER ;

CALL p1();

Notice, the 'a,b' value is ignored.
Inserting assigning 'a,b' to the variable a results in the numeric value 3, which is a bit combination of 1 for 'a' and 2 for 'b'.

Commas should be disallowed in values for SP SET variables.