It's impossible to send them to an invalid address, BUT it's entirely possible to send them to an address for which no one has the key.
Take for example: 1BitcoinEaterAddressDontSendf59kuE
Are you sure that this wallet does not have a private key?
Because it was created as a donation wallet by someone named holgero, and I can see a 0.559 BTC donation coming to it two months ago.