If you have a ladder rated for 500 pounds, it might not collapse with 900 pounds on it... But, it's always safe at 500 pounds.
Where I work, we make a board that runs off of 5V. There is about 20 chips on it, all rated for 5V. A few times I've accidently connected 12V to the power input. Usually the RAM chip will fry, and sometimes the CPU will fry. But the other chips usually survive.
Fortunately, I don’t have any experience with over-voltage on the Arduino! ... Yet.