Here are my exact repro steps

1. Clone madskristensen/WebEssentials2015@d946b8b (the version of WE shouldn't matter at all, unless I manage to fix F12)

2. Run

3. Open a Markdown file containing a C# code block:

   ```C#
   int x = x = 0;
   ```
   

4. Set Break on unhandled exceptions

5. Select any identifier defined in the code block and press F12

6. Roslyn will throw an ArgumentException (the buffer is somehow getting a StaticTextContainer; I'll file that issue after I manage to debug it)

7. Load symbols for the top two stack frames (EditorFeatures.Text & VS.LangServices) from DotPeek

8. Launch a third copy of VS, set Break On Exceptions, and attach to the original (debugger) copy

9. Open either of the top two stack frames

10. The debugger will crash with a null reference in vsdebugeng.manimpl.dll

11. Close the exception dialog in the third copy

12. Watch it crash too