Cables fray and the motor gears strip, which shows up as a window that goes down but not up, moves in jerks, or drops into the door.
Regulators fail in three ways: the cable frays and binds, the motor gears strip, or the plastic slides wear until the glass tilts. A window that goes down but not up is usually the regulator rather than the switch. Order the channel felt at the same time, because a worn channel is what kills the new regulator.
