Surprise reopening is one of the types of tap issues that is typical for the US government bond market.
The essence of this notion is that in case of after the end of the auction for new government notes, a coupon rate, interest payment dates, and a repayment date fully coincide with the given characteristics of a government note, which was issued earlier, the new note will be reopened or it could be said that the new note will be fungible with the old one. It should also be mentioned that CUSIP of a new note ceases to exist.
For example, after the auction of a two-year note with CUSIP 912828P61, it was found that the coupon rate and payment dates coincide with the five-year note CUSIP 912828UR9. After that, five-year note with CUSIP 912828UR9 was surprisingly reopened.