I walked into a fast food restaurant in New York City where a large poster in the front of the store advertised a new dinner special for $6.99. That was quite a bargain so I went to the counter and ordered it. Apparently the "special" was so new, it wasn't on the menu. At any rate the staff refused to honor the advertised $6.99 price, and computed an a la carte (sum of the parts) price that came to over $10. I declined to buy the special at that price.
I returned the following week, and the special was available, with a new posted price of $9.99. The staff would let me have it at that price (a small discount from $10-plus but not the bargain of $6.99), but for the same price, I ordered something else.
I didn't want to be an SOB the first time, but supposing I was, could I have legally held the restaurant to the posted $6.99 price for the special?