when one connection had done FLUSH TABLES WITH READ LOCK, some updates, and then COMMIT,
it was accepted but my_error() was called and so, while client got no error, error was logged in binlog.
We now don't call my_error() in this case; we assume the connection know what it does.
This problem was specific to 4.0.21. The change is needed to make replication work with existing versions of innobackup.