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.
No one has a computer or vanity generator powerful enough to create a bitcoin address with corresponding private key with such a long prefix in a timely manner. Someone just made an address and attached a proper checksum to make it valid.