When searching for international flights (YOW - FRA, for example), a little check box appears and asks if you want a flexible date search - meaning it checks a few days before and after your departure and return dates to give a matrix of airfares as a result.

I use Firefox on my PC, and every now and then that little check box doesn't appear and thus the website only returns flights for the exact dates I've selected. It's weird, and I don't know why it sometimes doesn't work.

I also occasionally get the option to search by schedule (which used to be an option on AC.com) that comes up with flights and schedules not found when searching by fare (which is now the default search option). However when searching by schedule it comes up with the cheapest economy option and you don't always have the option of selecting a T+ fare. A bit of a tangent.

Anyways, weird things can happen on AC.com.