Poloniex is pointing to the Double Spend problem which was recently rumored to be found in Tether currency. Double Spending was a problem or a flaw which was present in earlier digital currencies before bitcoin. Digital Currencies are just digital files which can be duplicated similar to other files in our PC. Satoshi was the first person to successfully solve the double spending problem prevailing in digital cash by introducing Proof of Work. Double spending flaw may possibly increasing the total supply of the currency causing inflation and panic in the market.
A blockchain research center posted a screenshot on Twitter claiming that they found a double spend attack in the Tether protocol when coins were subjected to double spending. This attack happened while user was sending currency to an exchange.
Here is the tweet about this vulnerability.
