Last year I saw an ad for a $99 item on sale for $79.  Went there to get one and the sign advertising it for $79 was still by the item.  Went to check out and it rang up as $99.  Clerk said sale went off yesterday, I said sign is still up advertising it on sale.  Employee checked and confirmed sign was still there.  Clerk honored the sale price without any protest from me.    

I always check the item price against the checkout price.  If buying multiple items I do an estimate in my head as to the approximate total cost and compare to the checkout price.  Don't worry about small discrepancies, only ones where dollars are involved.