Yes, ORD is the only place I've ever had the charge, taking a suitcase mostly full of books from home to school. I'm pretty sure that same suitcase was over weight other times and I wasn't charged - like lots of things, probably depends on which agent you get, how busy they are, and who knows what else.

Now I haven't checked anything on UA in probably two years, mostly due to shorter trips and more minimalist packing - so no worries about being overweight. I have wondered if they'd tell me I'm carrying on too much (often a backpack, messenger bag, and camera bag), but generally they don't bother, especially for people who don't fumble around all their carryons and are in seating group 1.

Domestic flights in Papua New Guinea had a strictly enforced 16 kg limit - that's total, checked and carry-on! Console yourself with that thought next time you have to overtip the skycap to avoid the $25 charge.