Off food past its use by AGAIN

I purchased some microwave veg at the fresh produce section, cauli broc and cheese sauce as well as one with butternut and zucchini. I got it on discount but coming home this evening an putting the one in the microwave to cook for supper I dished it up and it was completely off, bitter as anything and inedible! So that was a ruined supper. What gets me is the fact that, yes it's discounted, BUT on the packaging it was dated sell by 3 Sept and use by 3 Sept.... Why is this still on the shelf being sold of its past not just sell by but use by. I'm very disappointed and very disgusted as it's not a first. Please take this up with the store as it's wasting my time and my money. I have no confidence in the Somerset mall branch at all