it can also happen if you put milk in the freezer and forget to take it out. doesn't freeze entirely but becomes chunky like that but still not expired
i remember ludwig telling an entire story about this on stream once
Depends on when it's put on the shelves and who made it. I've seen my generic-brand milk have an expiration date 3 weeks after. but I finish it well before that. I imagine the fancy organic stuff would go bad quick though