I've had the same thing happen to me. Unfortunately, not even Oracle's support site has any information on the problem. In my case, I found that I was performing a backup and I had a automated job that would also periodically sync the recovery catalog or perform some other RMAN operation. All of these RMAN operations update the control file, whether you are using a Recovery Catalog or not. So make sure that this is the only RMAN process running.
If you do receive the ORA-235 error, your database is not affected, only your RMAN backup. You can restart the backup and everything should be fine.
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