Steps to reproduce the issue:
Run query below to generate parallel slave processes
select /*+ parallel(x,4) parallel(y,4) */ * from all_objects x, all_objects y;
Go to Database | Monitor | Session Browser to see the sessions.
Now highlight the main session and click "Cancel SQL for selected session ":
Cancel SQL for selected session with parallel slaves executes for all of them instead of main session (coordinator) resulting in ORA-00029 errors shown one after another in new modal window. Same thing with kill/disconnect( it shows ORA-00030 errors instead).