To repro:
1. Open project attached.
2. Open any scene and Open RestartGame script.
3. Add breakpoint on Press function.
4. Attach to Unity process.
5. Run Unity. Press Play.
6. Go to Monodevelop. Skip this method.
7. Press Pause execution.
8. Add comment into method and save script.
9. Go to Unity and trigger breakpoints again.
10. Detach Monodevelop ant return to Unity.
Actual result: You'll get errors in Editor. After some time Unity crashes.

KeyNotFoundException: The given key was not present in the dictionary.
System.Collections.Generic.Dictionary`2[GameManager+GameStates,System.Action].get_Item (GameStates key) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:150)
GameManager.Update () (at Assets/MonoPong/Script/GameManager.cs:97)

Reproduces in 5.3.30f1, 5.4.0b7.

Same problem happens with similar scenario in VS2015.