Using a simple project (probably any project) the Editor will crash if:
1) When the Editor session starts, VR is enabled and default Windows Graphics APIs is selected in Player settings.
2) In Player settings, toggle off VR.
3) In Player settings, toggle off Windows Default Graphics APIs.
4) Add OpenGLcore to the API list.
5) Delete D3D9 from the API list. The Editor crashes.

This set of steps consistently crashes the Editor. Skipping some of the steps or starting the Editor session with different settings often does not lead to a crash. So, this may be a corner case, but it is likely to rankle some of our customers.

I flagged this as not a regression because I can reproduce the crash in Unity 5.4.0b5.