Carlisle's Hallam Hope was sent off for a dangerous challenge on Darnell Furlong before the U's James Dunne was denied by Mark Ellis block.
Ben Williamson's tap-in for Cambridge was ruled offside and Derek Asamoah lifted over the bar from two yards.
Ellis then picked up a second yellow for time-wasting and was sent off.
Carlisle move down to 11th while Cambridge sit 12th, four and five points off the play-off places respectively.
Cambridge United were held to a goalless draw against nine-man Carlisle United, a result which damages both clubs' League Two play-off hopes.