this is kind of a funny discussion as ive seen this happen before, and yes you can flip the polarity of the magnets if the voltage is strong enough. had a guy use a jump box and did it somehow, even after rebuilding the starter with new brushes and cleaning the commutator it STILL turned backwards. of course the manuals and honda insist never jump off a bike or use jump boxes because the smaller wiring in the harnesses (and now the computers) cannot take the jolt of the high amperage coming from said practices. always take the battery out and charge it or replace it with a new one.
the other way to reverse a starter is like shadetree said, hook the battery up backwards. when replacing the battery make sure to buy the correct one, a YTX14L-BS battery (and its aftermarket variants changing the Y from Yuasa to any number of brands) is the same as a YTX14-BS battery in every way except for one important factor, L in the battery number means LEFT SIDE NEGATIVE POST. This is another common way to accidentally reverse the polarity of a machine by simply installing the wrong version of the right battery correctly.
one other way that is really comical as a side note, if you kickstart a yamaha YFS200 blaster and its in reverse, its because the flywheel key partially sheared and makes the engine RUN backwards.
the last example of something going backwards actually comes from John Deere, the flywheels are light on some of their diesel equipment, and it is possible to load the engine up so hard that the engine stalls. when this happens and if there is enough lash in the driveline from the axle splines to the transmission gears, when the driveline unloads it will sometimes spin the engine over backwards and even START the engine RUNNING BACKWARDS pulling air in from the exhaust and blowing it out the intake. most hilarious if you ask me, didnt believe it at first but had a tech show me a sooted up air filter my response was "SERIOUSLY?!" and much laughter ensued afterwards.