What can happen, and I know it occurs with anti-lock brakes on both aircraft and cars, is that if the computer systems receive a set of conflicting inputs, there can be a deliberate decision on the part of the software to drop out and let the pilot/driver do the work without software interference/assistance. As such, it's similar to option 2 from stickyb's post although it's not going haywire, it's deliberately giving up control.
If there's a bug in the software then it may do strange things if it receives an unusual combination of inputs, either by something crashing or (more subtle) causing an overflow in an intermediate part of the software that produces incorrect output. I think they lost an Ariane rocket to this one once.
...and what happens, when all the sensors receive the same information (pressure lines frozen with water, leading to wrong - but not abnormal - parameters messages to flight computers?