In RubyMine 2 RC1 (and earlier) on OS X ...
Open a project (A). From File | Reopen, open another project (B) and select 'New Window.' After it loads up, close project (B)'s window with the red x-button in the upper-left corner of the window. From project (A)'s window, re-choose File | Reopen. Project (B) is no longer listed in the reopen list and Project (A) is listed there instead, even though it is currently open. It appears to behave the same way if you choose File | Close Project.
In RC1, it also throws an IDE exception but window to 'blame rubycore' won't launch when you click the icon in the status bar. It did not throw an exception in the previous betas.
Ideally, a closed project (regardless of how it is closed) should always show up on the File | Reopen menu. Projects currently open should not appear there.