Absolutely correct. That's why I stated "discussion could be around extraordinary circumstances"
However, there is also EU ruling which states that in such case the buyer has to be notified IMMEDIATELY after such error has been found.
If this is not the case the seller has no right to contest the contract. Also then it would be quite clear that it was not an OBVIOUS mistake as it took very long for the airline to see it. Such then the buyer can not be made responsible to have known about the mistake (without stating it is a mistake)

With regards to EU261 I am not aware of any such case, where extraordinary circumstances can come in place for a question on pricing. Circumstances in EU261 are quite clearly described as ATC, Weather, Terror, Authroitive decisions etc. It might well be that EU261 and what you are describing are totally separate cases.