In 1989 or 1990, UA from DEN to IAD, brand new 757 just delivered from factory, Boeing reps on board advising pilots, mechanics and FA's. Pilot starts takeoff roll, speed accelerates (I don't know to what speed, although prior to nose rotation), the brakes slam, plane shudders, people scream, unsecured items fly. Plane diverted to side strip. Pilot announces that computer had determined flaps were not completely deployed and overruled cockpit to stop takeoff. After about an hour (with on-board Boeing reps conveniently advising), we return to runway to try again, and after acceleration...repeat of first abort. Another 90 minutes of investigation and pilot wisely announces "we're going to call it a day," and we return to gate and $16 hotel rooms (since no replacement a/c and last flights have already left).