Why would they sell me a quart of plain yogurt in a milk carton?!

If the carton says "milk" and it's a milk carton, you may have gotten a quart of milk that was imperfectly pasteurized. The flora and fauna that naturally exist in milk are killed in pasteurization, along with a number of enzymes. "Real" milk is subject to the kind of curdling you seem to be describing, as a particularly ambitious, concentrated group of (hopefully beneficial) bacteria will colonize a container of milk and make it "yog" (a phony verb for the process that produces yogurt, as in 'to yog').

You need to check the sell-by date on the carton, and if you bought it after its sell-by date, you should call the store, as it should not have been on the shelf. But if you bought it before its sell-by date, you really would be doing a service to the dairy to call them. Pasteurization kills bacteria, and clearly whatever 'yogged' your milk made it through. If all that happened with this batch is yogurt, that's fine. But cows' udders are near their back seats, and they swish their tails and poo while giving milk, and the main reason for pasteurization is to avoid E. coli issues.

So, really, if the milk was sold you before its sell-by date, you should immediately call the dairy and let them know they may have a batch of imperfectly pasteurized milk out there. Then call the store and let them know as well. They may want to pull it from their shelves.

It is very, very, very likely harmless, and I myself came from a farming community, grew up on, and love raw milk, and know of no instances of E. coli infections in all the years I drank it. But for some reason, the mechanized ways we produce things on the current massive scales somehow and counterintuitively seems conducive to spreading infections, and you would be doing other shoppers, as well as the producers and distributors and retailer, a huge favor by letting them know.

Maybe they wanted to save the trouble of having it in a different container, and also wanted to save the container. Maybe they love the environment.

no idea.better alert the better business beru of this sitution