To reproduce:
- Load the attached project in Monodevelop
- Load the attached project in Unity
- Connect the debugger to the unity editor

a) Press play in the editor.
b) Either hit return twice, or hit the Play button and then the next play button.
c) Monodevelop will stop at the breakpoint Client.cs line 139. Hit the Continue Execution button in mono develop.

If the game starts and the editor has not frozen, press stop in the editor and repeat steps A through C. It has been my experience that it will always lock up within three repeats of this cycle.