thread_statistics is a reserved word in 5.5
In PS 5.5 thread_statistics is a reserved word. Which means you have to quote it when using the system variable with the same name:
mysql> set global thread_
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'thread_
mysql> set global `thread_
Query OK, 0 rows affected (0.00 sec)
5.1 is not affected, the bug was introduced when porting userstat.patch to 5.5.