The current dispute between Aer Lingus and the lucky customers who purchased €5 business class flights to the US via their website has implications which reach far beyond the current situation. It is patently obvious that Aer Lingus hasn't got a leg to stand on (a wing to fly on) as there was a legally binding contract in place as soon as they accepted payment for the €5 flights on their website. It is irrelevant whether the offering was a mistake or not! The offer was made and accepted and money changed hands - Case closed! Hopefully the airline will fulfill the contracts made with the customers concerned but if not this looks like the ideal opportunity for a very costly class action which the airline will most certainly lose.
A much more serious situation will result however if the airline does not back down and if customers let them away with it! Contract law in this jurisdiction will be effectively worthless! Every time a party wants out of a contract all they will have to do is say "I didn't mean it - I was only joking - April Fool!" As anyone knows April Fools after the 1st April are invalid! I think it might actually be desirable that this does go to court regardless of Aer Lingus' response. Serious damage has already been done when a major company like Aer Lingus shows such blatent contempt for the law of the land! Consumer law is one of the vital protections within any civil society and any organisation that seeks to subvert it should face the consequences! If Aer Lingus walk away from this with only a telling off Justice will have been severly diminished!