[sillysaurus3]

If you try to decrypt C with any key other than the K that was used to encrypt it, you'll get gibberish (the decryption process will fail).

As far as I know, it isn't possible for an attacker to generate a message ciphertext C such that two different decryption keys would both decrypt to valid plaintexts. There can only be one valid decryption key; trying to decrypt C using any other key would yield random output, wouldn't it?

[kaeporan]

I hope I'm not mistaken, but my understanding is that you can have the same message C that would decrypt with K1 to the plaintext "Hello world" but with K2 to another plaintext of your choice ("Jello Warld" or whatever.)

[KMag]

This is only true for one-time-pad encryption. For a given AES-256 cipher mode and ciphertext C, there are at most 2 * * 256 possible decryptions of C. For most pairs of ciphertext and plaintext (C, M2) longer than 32 bytes, there doesn't exist a key K2 that decrypts C to M2. Even if K2 exists, finding K2 given (C, M2) with an average work factor less than 2 255 implies you've found a weakness in AES-256.