It's certainly not a big difference, depending on the codec from the video and which bit is different, it can range from 1 pixel out of hundred of thousands in 1 frame out of hundred of thousands in the video that is the wrong color or at worst, on older codecs, a series of pixels that are flipped for a second or two.
With audio, you either won't hear it at all or it will make a little out of place sound for a fraction of a second.
If you don't already have both files in you possession, you can't know if the 1 byte difference is real or not since different OS/software round up the values differently so unless you're 100% sure the testing condition are exactly the same for both files, you can't know if there's really a 1 byte difference between them.
And even if there is, you shouldn't really care since the chance you'll notice a difference are slim to none. If you can get it from the original uploader, than do it since it reduce the risk, if not, then it's no big deal and get what's available.
Back when I did fansubbing, the only copy we had of a couple of our oldest files had the wrong hash(meaning at least one byte in the files somehow changed since their creation), but we looked at them and didn't find anything wrong with them so we just used the new hash and re-released them like that. Unless you actually see a problem with it, there's no reason to care if it's a little different.
I didn't mean an added watermark when I talked about metadata, I meant that video file containers(mkv, avi, mp4, ...) have a section that contains data unrelated to the actual video and audio file. In this section, one could add his own name as the creator of the video file for example and this would in no way affect the playing of the video. All it would do is that with a player that can read those information, you could display them if you wanted to. You could even add junk in the container and as long as it's in the right place, it won't affect the movie at all other than the size.
For (an extreme) example, I could add a 2GB file containing only 0s in a .mkv and the video would play perfectly as intended and be exactly the same as if it didn't have that extra 2GB file inside.