SET @foo=bar;

This would cause a deadlock in mysqld, it now generates the error:
ER_SET_CONSTANTS_ONLY as follows:

mysql> set @foo=bar;
ERROR 1204: You may only use constant expressions with SET


include/mysqld_error.h:
  Added a new error message for SET
sql/share/czech/errmsg.txt:
  Added a new error message for SET
sql/share/danish/errmsg.txt:
  Added a new error message for SET
sql/share/dutch/errmsg.txt:
  Added a new error message for SET
sql/share/english/errmsg.txt:
  Added a new error message for SET
sql/share/estonian/errmsg.txt:
  Added a new error message for SET
sql/share/french/errmsg.txt:
  Added a new error message for SET
sql/share/german/errmsg.txt:
  Added a new error message for SET
sql/share/greek/errmsg.txt:
  Added a new error message for SET
sql/share/hungarian/errmsg.txt:
  Added a new error message for SET
sql/share/italian/errmsg.txt:
  Added a new error message for SET
sql/share/japanese/errmsg.txt:
  Added a new error message for SET
sql/share/korean/errmsg.txt:
  Added a new error message for SET
sql/share/norwegian-ny/errmsg.txt:
  Added a new error message for SET
sql/share/norwegian/errmsg.txt:
  Added a new error message for SET
sql/share/polish/errmsg.txt:
  Added a new error message for SET
sql/share/portuguese/errmsg.txt:
  Added a new error message for SET
sql/share/romanian/errmsg.txt:
  Added a new error message for SET
sql/share/russian/errmsg.txt:
  Added a new error message for SET
sql/share/slovak/errmsg.txt:
  Added a new error message for SET
sql/share/spanish/errmsg.txt:
  Added a new error message for SET
sql/share/swedish/errmsg.txt:
  Added a new error message for SET
BitKeeper/etc/logging_ok:
  Logging to logging@openlogging.org accepted