Just don't change planes/airlines at Chicago O'Hare. The left hand doesn't know what the right hand is doing there. We really got the run around on Sunday, being directed from one terminal to another and back again. We went through United Airlines security in London but after clearing cuss-toms in Chicago we had to be rechecked through security in one terminal, then boarded our US Airways plane in another, after clueless employees misdirected us twice. The third time we told the employees what was what.
Fortunately we had a two-hour layover to get it figured out. I recently booked our holiday flights on SWA and made sure to avoid changing planes in Chicago.