If an intern can bring down the whole presence of your company online it's not the intern who is dumb.
While that's perfectly true, I'm sure a smart and determined intern could cause an astonishing amount of damage in many many orgs. Hopefully not at Facebook though.
The difference is whether or not it was intentional. If someone can bring everything down accidentally, the system is broken.
Ditto if they can bring it down intentionally and cover their tracks so no one finds out who did it.
some startups would like to disagree
bingo
The dumb thing would be giving the intern permission to bring down the whole site.