Straight from CoinTelegraph!
SlowMist states in a tweet that they were able to send USDT to an unnamed exchange without correct field values on the transaction. This means that individuals may be credited for tokens without actually having sent them, leading to a double spend. Following the tweet from SlowMist, a founder of OmniLayer, the platform on which USDT was created, offered an explanation to the error:
ā[I]t appears that what happened here is that an exchange wasn't checking the valid flag on transactions. They accepted a transaction with valid=false (which they should not have), and then the second "double spend" transaction had valid=true, which they also accepted. Unless I am missing something, this is just poor exchange integration.ā
