This is a sign of a busted transmission. The shifting linkages inside of the must be jammed. Either that or some of the teeth on the gears must have broken off. The cause of this may be that it is just used a lot and it just wore out. Another possibility is not using the clutch or not using the clutch at the right time. If you are using the clutch then there may be a problem with the clutch mechanism to fail. This causes you to think that you are actually using the clutch when the clutch itself isn't working correctly, thus the transmission fails. This is a common problem with some older mowers. How the clutch works on most tractors is having an idler pulley put tension on the drive belt causing the transmission pulley to spin. When the clutch is pushed in, tension is released from the belt making it so that the belt doesn't grip the transmission. My guess is that the linkage between the foot petal and the clutch itself is broken or bent. There could also be dirt and grime in the clutch mechanism causing it to seize up.
Fill me in on any other info you find.