Banks and financial institutions typically have internal risk management systems that prevent occurrences like negative yields caused by manual errors. Traders therefore speculated that the system was manually overriden, which caused a negative yield quote.
The Clearing Corporation later in the day emailed traders to clarify that there was no change on its end to the way the system operates on inputting prices and the calculation of yields. Bloomberg News has seen a copy of the email. A bank placed a wrong price quote, which led to a negative yield as the paper was nearing maturity, according to people with knowledge of the matter, who asked not to be identified as the details aren’t public.