Twitter trick - you can ask your readers to tweet about your post, but in fact send a DM. Example https://t.co/03nZRdP2xO
— Egor Homakov (@homakov) December 14, 2013
But you know what's really cool? ANY app can send a DM on behalf of your account, by sending to API "d NAME TEXT". I just tested with Twitpic, as you can see it doesn't require any DM permissions.
Why is it a bug?
1) App is supposed to have Read & Write permission to access DMs. With this shortcut you can bypass that protection
2) DMs are easier to use for spam. User will barely notice it.
3) Also DMs don't show if it was sent with official client or a 3rd party OAuth client. Which is great for phishing.
[no permission] https://dev.twitter.com/docs/api/1.1/post/direct_messages/new
[warns about permission] https://dev.twitter.com/docs/api/1.1/get/direct_messages/show