Why does upgrading my smart contract let me silently corrupt storage that auditors already blessed?

Upgradeable proxy patterns (UUPS, Transparent Proxy, Diamond) are standard in production DeFi but carry a strict constraint: the storage layout of a new implementation must be compatible with every slot assigned by the previous one. Insert a new state variable at the wrong position and every subsequent slot is corrupted silently, with no on-chain error. Automated checkers like OpenZeppelin's upgrade plugin catch naive cases but miss ERC-7201 namespaced storage with custom packing, assembly-level slot assignments, and Diamond facet cross-collision where two facets claim the same storage region. A January 2026 peer-reviewed study (ProxyLens) found a significant class of inter-facet collision vulnerabilities that existing tools do not detect. Auditors review implementation contracts in isolation and cannot reconstruct the full storage map of a proxy with multiple delegated facets.