I've had an Arudino die "for no reason"... Maybe static discharge? Actually, it's running OK, but it's running a usless program and I can't re-program it (bad bootloader or USB/serial????).
At work, I've fried a few (non-Arduino) boards by running a 5V board from 12V. The problem is, we have a customer that puts the board in a box with a barrel connector that matches our our 12V products.
We have 12V "wall wart" supplies all over the place and in the "heat of troubleshooting", when you are switching things around and re-connecting things, it's easy to make a "little" mistake.
. Usually the RAM dies, and sometimes the CPU chip. (All of the other 5V chips on the board usually survive.) The CPU is in a socket, so that's no big deal. But but the RAM is soldered, so replacing it is a minor pain. (Of course we have plenty of parts in stock.)