When executing SQL in debug window, you can easily end in a state, where the SQL can not be executed and the session can not be terminated.

Steps to reproduce:

I have a test window containing the code:
I press F9 to debug and Ctrl-O to enter the code block.

I press the button 'Execute SQL in debug session...'

I enter the query:
and get the error:
This, I believe, is by "design" and is a known limitation in the database.

I open the debug session SQL window again and rewrite my SQL into:
and get the error:
I try again and get the error:
Sometimes I also get the error:
Sometimes is has helped to put a semicolon (;) at the end of the select to terminate the statement, but that does not help either.

This all ends in a loop that can not be terminated.

I can not break the debugger and eventually you will have to terminate PL/SQL developer via task manager to e.g. be able to compile program window.

We'll investigate.

Any progress on this?

We have not yet investigated this. It's still on the list.

Don't know if this is related, but after (successfully) running a debug session a query I execute in another window doesn't return with results and I can only stop this by closing the window, terminate session. After that I receive a ntdll error when I reopen the file and try to execute the query again.
I have also reported this weird query execute state earlier. Working with the beta version is kind of hard because of this bug as I'm trying to debug right now. I now have to close PLSQL Developer after every debug run.

After debug run I now just tried to open a clob in de query result of a query executed before the debug-run and now receive the following error:
Access violation at address 00007FFA08CD5CAF in module 'OraClient12.Dll'. Read of address 00000000149B4DD4

ntdll.dll error mentioned above is the following:
Access violation at address 00007FFA33D9D589 in module 'ntdll.dll'. Write of address 000000001720F7A0

@Marco, please address this issue before the final version 15 release