Very first flight I took, I was with my class and we were boarding our first flight. The FA stopped me and told me I'd have to check my bag because it was too big. She didn't have a measuring tape or anything like that and we were already on the plane (at the entrance) so I figured it'd be too late to check it anyway. It was one of those bags that come with the luggage set...the smallest one that, unlike the others, don't have wheels and is just pretty much a duffle bag type deal. Most of my classmates and teachers had already taken their seats.
Because it was my first flight ever and I was panicking I told her "this isn't my checked bag, it's my carry on"
and she told me "get everything you need out of it and go check it."
I replied "I need everything in it....it's my carry on!"
She wasn't impressed. But she said I could attempt to fit it in the overhead or under my seat and if I couldn't I'd have to go check it. Had absolutely no problem. It fit perfectly under my seat and left me all my leg room. Didn't have a problem with it at the Montreal airport or Frankfurt. No problems on the return trip either (and it was more full on the way back).
I think she was just being a bother.
Not sure if this even fits with what you were asking, but that's what happened to me. I think I will forever be nervous about what I bring as a carry on from now on....