Tweetbot for Mac, one of the more popular Twitter apps for OS X, is currently unavailable from the Mac App Store. That's led to some speculation about what might have happened. Since there's been no official word Twitter or from the developer, Tapbots as to cause, any and all of it remains just that: speculation.
We'll update if an when more information becomes available. Until then, if you're trying to buy Tweetbot from the Mac App Store, you'll just have to sit tight.
Best case it's just a super annoying glitch somewhere and Tweetbot for Mac be back up and available again soon.