Then you can imagine it's OK! Or you can imagine you've fried your imaginary Arduino! 
You can also imagine that it reads zero with negative voltages and that's actually true as long as you don't fry the thing.
There are internal "protection diodes" in the chip to protect against voltages higher than 5V, or negative voltages. But somewhere I read they are only rated for 1mA so it's easy to fry those and once they fry the whole chip could go...
Here are some protection circuits. Note that they ALL have current limiting resistors. This protects the diodes as well as whatever is connected from excess current, if the resistor is chosen properly. In many cases the 100-Ohm resistor is too low for whatever is connected.
If you use Schottky diodes they will "turn on" at al lower voltage, before the internal diodes start conducting, so the internal diodes are protected.