The one where you buy something on eBay and it never arrives.
You contact the seller and they say 'Oops! We never actually had that item.'
They refund your money, but the item is still listed for sale.
You leave them fair and honest feedback and mark it as 'negative' because that is what the whole experience has been.
You notice that there are a LOT of other comments there saying similar things.
Then they start emailing you every three days practically begging you to change the feedback to positive.
Yeah, good luck with that.