See of you "get" this. Twitter apparently won't let you tweet a little tweet if it starts off with the word "get."
John Herrman on Buzzfeed points out that typing things like "get better, get lost, get away, get inside, get out, get asdfgjkdjghlsldskf" will make your tweet disappear the moment you try to post it. Gizmodo coins this flaw the "get glitch."
Herrman points to a potential reason cited on StackExchange for the "get glitch." It appears to be left over from Twitter's old SMS interface. One of the SMS commands was "Get [username]". This would send what ever your action was to that person.
I tried a "get" command -- "get it done" specifically -- and this worked. "Get better" did not work. So, this confirms it is a two word "get" tweet that gets stuck.
There are other interesting commands that won't necessarily complete your tweet but will perform other functions too. Texting "help" or "info" to Twitter won't post a tweet but will actually provide you with tips. "Fav," "fave," and "favorite" will mark something as a favorite item.