I've also had a number of experiences with food from Sainsbury's (always the FH branch) that deteriorated well before its due date. At first, I assumed I was just unlucky (after all, these sell-by dates are based on pretty inexact science) but it happens so often now that I'm beginning to supect them of a little surreptitious re-labeling.
Other favourite ploys I've noticed recently include putting things on a Special Offer shelf when they are not actually on offer. For example, the clementines are on special offer but the box is actually full of more expensive satsumas; or vegetable soup is half price but the 'Half Price' label is placed in front of the chicken soup which is full-price. (How carefully do we check our till slips? I now do it routinely and it's amazing how often I end up at the Customer Service desk because someone has 'made a mistake'.) These ploys are sneaky rather than illegal (unlike the scam I mentioned at the beginning of the thread) but at a time when many families in Forest Hill are trying hard to stretch the budget a little further, this practice of pretending that items are reduced when they are not strikes me as pretty contemptible; every bit as bad as the shoplifters they complain about. (And no, Brian, I'm not defending them either!)
Incidentally, I didn't report the store to Sainsbury's because I fear they will either do nothing or, worse still, make a token gesture by disciplining the store manager or whoever re-labeled the items. In truth, I suspect the problem lies with senior management and a company culture that sets impossible targets and doesn't really want to know how they are achieved.