Replays are broken all the time. It's almost always
not the user cheating afaik. Replays break when something is very close. Meaning, on harder maps the likeliness of finding a broken replay is higher. More suspicious would be a missing replay, though that happens sometimes too. Don't worry about it, happens all the time. (At least, in CTB. I don't do normal osu! but I imagine that the same thing would happen.)
EDIT: Wow adam, you're fast

and I didn't know about that thread.