To reproduce:

1. Download mine attached project "coroutineOverload.zip" and open in Unity
2. Open "coroutineOverload_test" scene
3. Select "Sphere" gameObject in the Hierarchy and set it Tag to "Player
4. Attach "overloadScript.cs" to "Sphere" gameObject and set "Player" reference to "Sphere" gameObject
5. Open "overloadScript.cs", uncomment #20 line, comment #22 line, save the script and return to Unity Editor
6. Enter Play mode
7. Press "W", "A", "S", "D" keys on your keyboard and you can see that Unity Editor is still interactable
8. Exit Play mode
9. Open "overloadScript.cs", comment #20 line, uncomment #22 line, save the script and return to Unity Editor
10. Enter Play mode
11. Press "W", "A", "S", "D" keys on your keyboard and you can see that Unity Editor froze and sometimes crashes

Notes:
- Stack trace is shared in comments section
- This issue appears on both Windows and OSX

Reproduced on Unity 5.6.4p4, 2017.1.2p3, 2017.2.1f1, 2017.3.0f2 and 2018.1.0a7