Mother fucker. I just had to dump out an entire bowl of cereal cause the milk I just opened for the first time, with a sell by date of 12/12 on it, tastes spoiled.
I've had this happen before but not 4 days earlier than the date, but 2 days, and on the date of. Wtf? It ain't my fridge temp either, that's fine (one of these times it happened at work from their fridge too).
It actually seems to occur when the milk is left sitting for an extended period, not opened and used now and again. Like if the milk was opened after purchase and a cup of it was used, and then it sat unused until a couple days before the date, or if the milk was never opened and like today it sat until I opened it a bit ago. I dunno it probably makes no sense but that's the common denominator between my experiences.
F u milk