I'll never forget watching a motorcycle reverse into a car at a stop light! The poor rider's bike had stalled; he went through the normal start procedure (check fuel, ignition on, out of gear) and gave the kick start lever a hefty kick. The bike fired up and sounded normal. As the lights changed he selected first gear and, much to his (and the car driver's) surprise, went backwards!
The cause of this problem with 2-strokes is the ignition timing. If the timing is close to TDC (top-dead-center) it is possible to catch the piston just at the wrong time with the result that the engine runs backwards.
This problem can only happen on a 2-stroke because there are no valves to be operated in a set sequence, as in a 4-stroke. Typically, this problem would happen when the contact points became worn, or more precisely, when the contact point's heel became worn. The net effect of a worn contact point heel is the ignition timing progressively becomes retarded.
Checking the ignition timing on an early motorcycle is best done on a more regular basis than their modern counterparts; monthly if the bike is ridden daily such as a commuter bike. Not only will the potential for running backwards be avoided, but the engine's entire performance range will be optimized too.