the cause of these frightening close calls. reporter: the close calls involve air traffic controllers on each end of the country. may 9th in houston, united flight 601 was departing from vancouver on runway 9 as united flight 437 was leaving from mexico city on runway 15. the controller apparently made a serious mistake. 601, thank you, heading 340. instead of telling the vancouver flight to turn left he told him to turn right into the path of the other plane. the controller then quickly realized his mistake. united 601, stop your climbing and stop your turn, united 601. reporter: the planes came 400 feet vertically, .8 miles laterally. seconds away from disaster. arriving united 737 flew right over the top of a departing express jet regional flight that had been cleared for takeoff on an intersecting runway. do you have him in sight? maintain visual. oh, yeah, he s real close.