I can't remember exactly what it's called - I assume it has to be an "obvious" error.
Sometimes companies honour the error for PR reasons, there was a John Lewis situation I think where they did this recently.
Advertised prices are "invitations to treat" rather than binding contracts of sale.
Ah - precisely - I'm so glad you brought that up. If only more people understood this simple principle!
You're right though, frequently honoured for good PR - it's rare that an item going through checkout at the 'wrong' price perhaps due to an expired offer is not adjusted.
This prevented, explicitly including the case of "mistake" fares, the post-purchase increase in the price of airfare. This was done to prevent airlines from post-facto declaring a promotion a "mistake" when deemed "too successful" later.