From what I've seen, most stuff is either unusual flavours that didn't sell well (like those Herbert Adams "Asian" meat pies) or closer to the best before/use by date than what you'd find in a conventional supermarket. For example chips might have a best before of 3 - 6 months or something in a bigger supermarket, whereas they'll have 1 month left at NQR. Actual out of date stuff is very rare, at least in my experience.

I agree that it can be a bit of a trap because prices are so low, which is why we only go there at the end of a full shop so we know what we need and what our remaining budget is (we withdraw cash every fortnight and use that as a hard limit on spending, so the NQR budget is usually the leftovers from that). Sometimes the prices aren't actually competitive, or they just don't have the stock, so it's useful to go in with some idea of what you'd otherwise be paying.