Tried only with Web Player.
Nothing bad actualy happend.

Steps:
1. Go to File -> Build Settings
2. Select Web Player in list of platforms.
3. Press Player settings button.
4. In player settings change Default Screen Width to -100
5. Press Build and Run button.

Expected:
Minimal for resolution values must be 1

Actual:
It will work in some default resolution or something like that.
If you input 0 for Google Native Player it will display error.