I guess I'm guilty of being "sneaky" as well.

I was in first class on a 757 a couple of weeks ago. I didn't board early because I arrived at the airport very late. By the time I boarded, the overhead bins were all full. My seatmate told me that one of the large carry-on bags belongs to some guy back in coach who just stuffed it in there. I pulled it out and told the FA that I wanted it gate-checked. I then put my own carry-on in the overhead.