So I have been doing a rather large amount of HITs, but I just got my first rejection. I accidentally submitted a blank survey to Kyle Dopp (AFAIK), but that is still pending approval. My HIT was rejected by UCBHSB Research. In the feedback they told me this: "We only received 67 of our requested 70 surveys. This led us to find the three people who entered fake codes. You were one of those three." I have submitted 367 HITs, 332 approved, 1 rejected (this one!), and 34 in pending. I wouldn't even know how to get a fake code, let alone try to cheat them out of 10 cents!
I'm guessing that the data can only be saved once per code. They just assumed that you had used a fake code since they did not receive any data connected to you. Respond back to them and explain to them that you accidently submitted a blank survey just incase they are planning on blocking you. I'm new to turking, but from what I understand, a few blocks can get you banned.