Gmail has this great feature where you can undo sending an email within 5 seconds. It's in Labs, but I turn it on for every account. Every week, I undo send for whatever reason. Never out of regret, but just because I might have missed something. I'd like that for Twitter. Countless times have I tweeted a typo and a few minutes later discovered that fact. The other blunders have been replies where I press send by accident when the tweet is not fully-formed or, even worse, has a typo. For tweets that don't need to be sent right away, I'd like a client to show it in my timeline as if it were published but with maybe an indication that I have a few seconds/minutes to change my mind. It wouldn't actually post until that time has elapsed. Twitter's most frequently requested feature is typo correction. I can with 100% certainty they will never add that feature. Think of the implications. But clients can add an "Undo" feature before it's actually sent out.