In a normally functioning transmission, the car stays in the gear you designate, or that the computer designates for a given RPM range, until you or the computer initiate a gear shift.
But on a transmission in which the gears slip, the car can spontaneously pop out of the gear it's in while driving and (in a manual) force the stick back into neutral [source: Procarcare.com].
This is unnerving at best and potentially dangerous at worst: when you mash the gas pedal to avoid an out-of-control vehicle, the last thing you want is a transmission that doesn't get power to the wheels. No need to scratch your head over whether this is trouble or not: if it happens, you know it's time to have your transmission examined.
Our next trouble sign might seem like a real "drag" if it happens to your vehicle, but its fix is often not so complicated -- see what it is on the next page.