if that happens i would recommend talking to a manager. you can't have two prices on the same exact item. that's illegal.
The only way it would possibly be illegal is if you could argue bait and switch. Effectively, you try to get a game like Borderlands 2 during the time of the sale for $25, but they say that one is "sold out", yet they got the copy on the shelf they tell you it is full price. While it is not a different product, it is still a higher priced item than what the ad states. Even though it says "limited quantities", if the particular item is still out on the sales floor, then theoretically it should be sold for the listed price.
If they really don't want to sell more than 5 copies or whatever amount of a product during the sale, then they should take the excess product off of the sales floor. Unless they do, people have a legitimate argument, especially for the games listed in the ad. There are no special SKU/UPC numbers in the ads, they only show the covers for the games and the price. If you want the game in the ad, if they have it on the sales floor and during the time specified for the sale, they should give you the advertised price.