Travelling with my local national carrier this matter is never a problem. If there is the slightest doubt about the cabin bag (even if it is legal size) it is labelled in a polite manner and taken from me at the gate or bottom of the steps. It s then returned as I get off the a/c so no delay at baggage claim. 2 pieces allowed so a computer bag and coat are kept with me in the cabin. If the cabin is not full all my luggage will be inside. This problem can be caused because the carrier has different types on short haul and some overhead bins are smaller. Problem solved in a calm polite manner.
Regarding easyjet and AGP. It was a few years ago, but I saw a group of guys, at the gate, kicking and breaking the wheels off a carry-on so it would fit in the template. Another case was where a lady had a smallish wheely carry-on but the nazi style young gate girl would not allow it on; judging it by eye and not a template. The poor lady had a large plastic carrier bag. She stuffed all her contents into this and left the wheely. Unbelievable, unacceptable, disgraceful.
Surely it has changed, but some seem to suggest the jury's out. It takes so little to achieve so much. YES YOU CAN.