I was recently made aware that several Bitcoin side-systems in development were vulnerable to faked SPV proofs using 64-byte transactions. The impact varied from one side-system to another, but for some it was as critical as “an attacker can drain all the funds from the side-system”. This is a perfect illustration of the footgun concern: people don’t know about this obscure weakness and go about creating systems that rely on SPV proofs without the equally-obscure mitigation. It also illustrates the complexity concern: those proofs are often implemented in constrained environments such as smart contracts. The case for making 64-byte transactions invalid is stronger than i initially thought.