I once bought three different three-for-two offers in the UK, expecting to get the 3rd, 6th & 9th most expensive items free, but I was actually charged for all but the 7th-9th, which cost me a lot more

(I got the price I expected in the end, by making three separate purchases)

I also saw a competing chain labelling food nearing its use-by date with a higher price than otherwise identical items with longer to go, but presumably that was a genuine error