[JadeNB]

It is possible to enter completely fake information, rather than someone else's address. (For example, aside from the burden on the post office, I think entering a street address for an apartment complex without an apartment number harms no one.)

[jrockway]

Though unfortunately the mailman is usually smart enough to route that kind of mail if the name on it is correct.

Actually, it's quite difficult to fail to send someone a letter. The post office does try pretty hard to deliver mail.