Elephant diffuser
BitLocker's original add-on to AES-CBC, stirring each sector so that a single flipped bit garbled the whole thing.
In 2006 Microsoft's Niels Ferguson published the design for BitLocker in Windows Vista: XOR with a per-sector key, then two unkeyed diffusers, A and B, that mix every word in the sector with every other, and finally plain AES-CBC. The goal was to approximate a cipher with sector-sized blocks. With bare CBC an attacker can flip a ciphertext bit and get a predictable flip in the plaintext; with the diffuser in place they just get noise.
Ferguson was upfront that the diffuser came without a formal security proof. It also wasn't FIPS-approved and cost performance, so Windows 8 dropped it in 2012, and Windows 10 version 1511 introduced XTS-AES, today's default. Strictly speaking that was a step back on malleability, traded for a step forward on standards compliance.