Zcash, before the Sapling network upgrade (2018-10-28), had a counterfeiting vulnerability. A key-generation process, during evaluation of polynomials related to a to-be-proven statement, produced certain bypass elements. Availability of these elements allowed a cheating prover to bypass a consistency check, and consequently transform the proof of one statement into an ostensibly valid proof of a different statement, thereby breaking the soundness of the proof system. This misled the original Sprout zk-SNARK verifier into accepting the correctness of a transaction.