I pulled an unopened gallon of milk from the fridge and checked the date on it. Its label says that it should be sold by September 9th of this year. I got myself a glass and opened that fucker up and found that it was full of lumpy milk.
Presumably it was not kept chilled at some point in the supply chain. Unless sterilised, milk already contains populations of bacteria capable of turning it into yoghurt or lumpy, stinky hell - it only takes a few hours warmth to provide the conditions for rapid growth.
It probably got left out of the dairy case for awhile. I cringe anytime I see milk or meat left on a grocery isle by inconsiderate shoppers. It may sit there awhile before a stock boy puts it back in the meat or diary case.
If your milk is curdled then it must have been at room temp for quite awhile. I’ve brought home bad milk a few times. It’s not fun missing breakfast because the milks bad.