ORA-00031 Error -> Session marked for kill

This explains the oracle error, ORA-00031 in detail, the possible reason(s) and steps to solve the same in Oracle Database.

Oracle Error: ORA-00031

Description: Session marked for kill.

Reason(s) for this Oracle error: This error will be raised when you try to kill a session by using ALTER SYSTEM KILL SESSION command, which is already marked for kill.
The delay in killing could be either because of a network operation or it is rolling back and the session will be killed as soon as possible after the current uninterruptable operation is finished.

Possible steps to solve this error:
1) If you want to kill the session sooner, issue ALTER SYSTEM KILL SESSION command again.
2) No action is required in reality if you are not in hurry to kill the session.