Update: I got a question by mail if this bug is in the Gold release. It is. I'm running Revision 20081211.1925 (Release 8.5).
When debugging code that fetches the active NotesUIDocument from NotesUIWorkspace, NotesUIWorkspace.CurrentDocument randomly (?) returns Nothing. Sometimes the client also crashes when running code that does the above.
A couple of colleagues that work exclusively on the Client have gone back to Notes 8.0.2 because of the problem. Code that's affected by the above problem in the standard client seems to work when running the basic client.
I use this method to run the Basic Client.
A thread about the problem in the Notes 8.5 forum
If you have the same problem, please join in the discussion.