You need to go to a Dairy farm and see how cows are actually milked. You will realize its almost impossible for anyone to mix milks there.
It isnt like the worker in Dunkin Donuts switching milk containers.
You must think that a milk maid sits under the cow and milks it, If that was done then it would be possible to mix it up. But thats not how its done.
You cant lie and hide something, there is no easy way to put anything in it. The Milk is basically unexposed to the air until you pour it out of the Milk Carton. Its all done by machine.
A Machine Milks the cows, The Milk is put in a large vat where its pasterized and Homegized. The milk is then pumped into a large truck (That looks like an oil truck) and trucked to the packaging factory, The milk is then pumped out of the truck into the packaging machine where the milk is poured into the Milk Cartons,