I have found some odd things sometimes but nothing really significant apart from 2 years ago when they were restocking the ice cream shelves with ice cream that had been taken out of the freezers as they had broken down, stored in a cold store( it had defrosted and was kept at temps higher than freezing) and then put back in the ice cream freezers in the shop floor to refreeze. I found this out when I went to buy some and found it to be completely melted. I was pregnant at the time so was outraged to find them restocking the shelves with this defrosted stuff- it could have caused serious illness. There was no real attempt to conceal it either- the staff confirmed everything and said they were instructed to do it.
I stayed to watch as the shop assistant unloaded the shelves again, moaning at me for making his life harder. hopefully they really did ditch the stuff but after that I am very careful now about dates etc. I did formally complain to both trading standards and Sainsburys head office but never found out the outcome.
I have frequently buy chicken fillets only to find them stinking out the house the next day due to bacterial growth due to poor storage conditions. Its not good enough but I suspect that it happens in most supermarkets.