It's against the law for them to cancel the order and not enforce it upon demand, even if they "reserve the right to," after you have received confirmation. Companies do it anyway, because most customers won't complain.
If your order is canceled, I would call customer service and demand they enforce the sales contract. If they point you to any "we reserve the right to cancel order, fix prices, etc." provisions, these are almost universally considered unconscionable by a court, and are never enforced. If they refuse, just say you will file a small claims suit for specific performance, and that usually will get you your order.
That's assuming one wants to deal with the company on the phone for however long it might take. In the end, though, they have to sell you what they sold you. Often times, it isn't worth the aggravation, hence why companies are often able to successfully back out of legal contracts.
It happened to me once on
Amazon when I purchased something that I didn't even realize was a price mistake (and while it was a bargain, it wasn't one of those "obvious" bargains). They tried to cancel my order, but a quick call (admittedly Amazon has great phone customer service) fixed that up real fast. Amazon even admitted to me that they know they can't legally cancel the sales agreement, but they try to with customer anyway. In basic contract law, if a customer doesn't take any affirmative action to have a reneged contract enforced, the customer is considered to have ratified the cancelation of the contract. And, when the customer agrees to back out of the contract, the store has not done anything illegal.
They can't do anything now that I have them right?
