What could be going on here? On several occasions recently, I have purchased small (half pint) cartons of half-and-half (10%) cream. The "out" date on the carton is at least four weeks into the future. My fridge keeps it about 2 deg C (35F maybe). Freshly opened after maybe a couple of days in my fridge, the product is already curdled. I often only see it when I pour it into my coffee, but I've also seen it already curdled in the carton (starting to look like cottage cheese). What could be going on here? Is this an issue I need to take up with the grocer? Is it a product quality problem from the dairy? Have I handled it improperly somehow (fridge too cold)?
If the date on your carton is more than four weeks out, the product is ultra-pasteurized and should definitely last more than a few days. As long as your refrigerator is not cold enough for the half and half to freeze, and you've not kept it out too long, it's not likely that it's anything you have or haven't done.
That said, it's most probable that somewhere in the supply chain the product was not handled properly. But it's difficult to know where the breach occurred.
It may have been kept out of refrigeration too long when a delivery occurred at a warehouse or store. Or a customer may have decided not to buy it and set it on a shelf to be discovered later and placed back in the cold case.
There are any number of things that can happen between production and purchase and, unfortunately, we have no control over them and have no way of knowing what actually occurred.
I would definitely let your grocer know of the problem. He/she may be able to find and fix the problem if it's an ongoing issue.
It sounds like normal dairy spoilage. The date on the carton refers to the sealed product, which is usually processed in a nearly-sterile environment. Once you open it, you have 3-5 days to use it up.
If we do the math, you would need to add a tenth of a pint to your coffee daily to use up a single carton in 5 days, that's 1.5 times as much cream as a traditional espresso cup, or a sixth of a pint to be through in 3 days. I suppose only very determined coffee drinkers get to these numbers. So, with typical usage, you would expect to go over the safe range for each carton you open.
It is likely that other types or brands of dairy don't show visible spoilage that soon, so you could have developed different expectations. But first, not all bacterial growth is noticeable as spoilage, especially ESL dairy can harbor very high bacteria loads without noticeable changes. And second, food safety limits are based on a worst case calculation, so many of the other dairy products could simply have low bacteria numbers at the fifth day, and so not show spoilage. This could have contributed to you intuitively assuming your cream should still be good after what you call "a couple of days" (have you actually written down how many they are?) while in reality, spoilage is to be expected at that time.
If you are seeing spoilage before the third day, this is indeed a sign that the milk has been stored improperly. The too-cold fridge cannot do that, so it must have happened before that, and you could in principle talk to the grocer or producer. I cannot say what your realistic chances of success are, though.
As logophobe's comment says, this assumes that you are seeing actual spoilage, and not fat separation. If you are not sure which one it is, it is best to have somebody who has seen both look at it.