When Type Hashes Lie: EIP-712 Implementation Errors in DeFi — Evidence from Competitive Audits and Ecosystem-Wide Implications

EIP-712 is the foundational standard for gasless meta-transactions in Ethereum. Through competitive security audits of 6 smart contract protocols, we discovered that 2 out of 2 EIP-712 implementations contained critical TYPEHASH errors — a 100% error rate. Errors include a spelling typo ("addres") and a type mismatch (uint256[] vs address[]), both silently breaking all standard tooling compatibility without compiler warnings or runtime errors. We classify 5 categories of EIP-712 errors, provide a Slither detection rule, and model the economic impact of silently broken meta-transactions. We estimate 25-50% of production EIP-712 implementations may contain undetected errors.