Slave fails when switched from STATEMENT to MIXED and receives ROW event

The following is probably an edge case, but it's still a bug. Tracking it in the code may yield a key for other latent problems.

Servers started with binlog_format=STATEMENT.
log_slave_updates is enabled.
Changed SET GLOBAL binlog_format='MIXED' on servers while running.

Later, we see this error.

140930 8:51:33 [ERROR] Slave SQL: Error executing row event: 'Cannot execute statement: impossible to write to binary log since statement is in row format and BINLOG_FORMAT = STATEMENT.', Gtid 0-9-360301963, Int
ernal MariaDB error code: 1666
140930 8:51:33 [Warning] Slave: Cannot execute statement: impossible to write to binary log since statement is in row format and BINLOG_FORMAT = STATEMENT. Error_code: 1666

That of course shouldn't happen, as the binlog format has been changed.
Only a mysqld restart (with binlog_format=MIXED) resolved this.

Conclusion: some variable in running server didn't update to MIXED.