I know here in Ontario, they can cancel but they are legally open for violating the consumer protection act for misleading practices and a mistake is no protection in court.
An agreement between stores, and I believe one Amazon.ca upholds, is that they offer you the product at the original price with a $10 discount/credit (if it costs more than $10 - free if it costs less than $10). However, this is just a practice to placate the consumer as on gross mispricings like a $1000 TV for $5 people know they're taking advantage and will either be happy to take the $10 off, or will cancel the agreement thus removing all legal liability on the company. However, if the $5 had entered the till and a manager came running out trying to stop you leave with the TV, well you just got a $5 TV and if they stop you, they get up to a $250,000 fine and you get yourself a TV. Quite ironically though, if you notify the cashier of the mispricing, most store policies will give you the item for free regardless of price! So the people walking away with the $10 credit are really losing much more than if they had honestly notified a manager.
One of the caveats is that the agreement is generally to sell X number of products at $Y price, and if they don't have the quantity of products to supply the amount of orders they can actually cancel the orders and later reoffer them at a corrected price, which is probably what Argos got away with.
