I recently came across a signature check that was (correctly) checking the signature against a public key... The issue was the public key itself was unauthenticated, and provided by the (signed) ciphertext itself... Meaning the crypto was fine, but it wasn't checking anything meaningful, as any rogue message would just include its own public key in the message!

It's not only about the raw operation of checking bytes are equal (hopefully in a constant time manner, if applicable), but also about ensuring the desired security properties are actually present in the application!