When you try to decrypt something using AES_256 or similar, using the wrong decryption
key, the result is cryptographic noise. Cryptographic noise is data that might as
well have been randomly generated, as it is not even close to the original, no matter how similar the decryption
key you put in is to the correct decryption
key. This is as a result of the strong diffusion and confusion principles employed by modern encryption standards like AES-256. Even if youre only slightly off from the correct decryption
key, say you forgot to capitalise a letter, you will never get a "partially correct" or "degraded" version of the original data. It will generate what one might describe as "corrupted, meaningless
garbage, taking up space on your computer."