Why arduino can fry?

krupski:
FWIW... I've accidentally shorted outputs and even inserted the 328P backwards... never hurt the chip (yet).

Damaged components don't always fail immediately after being damaged.

dhenry:
People screaming about "damages" here simply have no idea what they are talking about; or don't understand the datasheet when they quoted it as support.

I'm bookmarking this as a great example of "the pot calling the kettle black."