Best Buy has a secret intranet site that it can reference when you come in looking for your 'best' price. Their site doesn't necessarily reflect the same prices as you'll get in your web search. You can read more about it here:
http://www.engadget.com/2007/03/03/b...-site-exposed/
FWIW, something similar happened to me last year, but at Staples. I had some discount cards from them and happened to be in the market of a larger hard drive. Doing some searching, I found a 200 gb Maxtor that met my needs of price and size and printed out the page to bring with me. While I found the same hard drive there, the price on the shelf was substantially higher. I brought it over to the cashier, and had her scan the price. Sure enough, the price was the shelf price and not what I'd printed out. I had the manager called over who pulled up a Staples web price but it wasn't the same as mine. However, after a brief verbal haggle, just barely loud enough to make people stop and listen for a bit, he said he'd honor the web price but offered no explanation for the discrepancy. He also honored the discount coupon I had. I didn't think much of it until the Best Buy thing came along and I'm curious whether this is more widespread among retailers who also maintain a store presence on the web.