A transaction could result in having an extra event after a query that
errored e.g because of a dup key. Such a query is rolled back in
innodb, as specified, but has not been in binlog.
It appeares that the binlog engine did not always register for a query
(statement) because the previous query had not reset at its statement
commit time. Because of that fact there was no roll-back to the
trx_data->before_stmt_pos position and a the pending event of the
errorred query could become flushed to the binlog file.

Fixed with deploying the reset of trx_data->before_stmt_pos at the end
of the query processing.