Production post mortem: The opener of files and destroyer of peace

A customer opened a support call about a recurring problem that they had in their system. System.InvalidOperationException: Errno: 1224='The requested operation cannot be performed on a file with a user-mapped section open. ' (rc=0) - 'Failed to rvn_write_header 'headers.one', reason : FailOpenFile'. FailCode=FailOpenFile.