Horton principle
Authenticate what is meant, not just what is said — a MAC should cover the whole interpretation of a message, not merely its bytes.
Niels Ferguson and Bruce Schneier stated it in Practical Cryptography in 2003, borrowing the name from Dr. Seuss's elephant Horton, who meant what he said and said what he meant. The point is that a valid tag over the wrong data is worthless: if the algorithm choice, length fields, protocol version or sender are left out of what gets authenticated, an attacker can change the interpretation without touching a single MACed byte.
Plenty of attacks are violations of the principle in disguise — downgrades where the cipher suite was never authenticated, or concatenated fields that can be split two ways. The cure is dull and effective: unambiguous encoding, and every bit of context inside the MAC.