Denied at the gate (YVR)
Managed to make the "final boarding call." I showed my ID to the gate attendant and at the last second they wanted to check if my backpack fit in their luggage sizer. In the middle of trying to make it fit they recommended I pay for it. I declined and said it'd fit and after arranging a few items that gravitated to the bottom of my bag it worked just fine. I showed the attendant and they told me it was now too late to get on the aircraft despite there being 25 minutes left before departure.
It would have taken longer to process a payment than to rearrange items in my backpack. I think they just wanted to stick it to me for not paying for carry-on.