I hate to sound stupid, but how would I NOT maintain ownership of the address you sign with?
I guess it is probably unusual, but I've sold entire wallet files. Probably not relevant to this topic.
A much much much more common way is to have a wallet file stolen from your computer.
Even more common than that would be to sign up for an on-line wallet or other Bitcoin related venture and have the proprietors fold up shop with the secret keys that the users (incorrectly) considered their own
That has been repeated ad-naseum since at least right after I got actively involved and the 'mybitcoin' scam went down.If you send it directly from MtGox or Coinbase or some other exchange that maintains the private key to the wallet holding your coins, you won't be able to sign it.