It's a regression issue.
The reason of the bug appeared to be an error introduced into 5.1 source code.
A piece of code in Create_file_log_event::do_apply_event() did not have test
coverage which made make test and pb unaware.
      
Fixed with inverting the old value of the return value from 
Create_file_log_event::do_apply_event().
The rpl test suite is extended with `rpl_cross_version' the file to hold 
regression cases similar to the current.


mysql-test/suite/rpl/r/rpl_cross_version.result:
  new results file
mysql-test/suite/rpl/t/rpl_cross_version-master.opt:
  options to the server to be able to start replication to itself
mysql-test/suite/rpl/t/rpl_cross_version.test:
  regression test for bug#31240.
sql/log_event.cc:
  Correcting the return value from  Create_file_log_event::do_apply_event()