We experienced this error today when a tester was modifying records on our test server but forgetting to commit his changes. This has the effect of records being locked and unmodifiable in the database.
SQL Error: 1013, SQLState: 72000 ORA-01013: user requested cancel of current operation
Our application will cancel any transaction that it deems to have timed out. In this case it was attempting to update records which were locked and not released by another transaction
The solution is the educate testers/developers to be careful not to lock records in the database for extended periods.