A potential perpetual motion machine? If I were to freeze a superconductor, the ones made up of yttrium barium copper oxide, with liquid nitrogen and placed it on one of those strong neodymium magnets, it should float right?
Now what would happen if I made a Mobius strip of steel and lined it up with those magnets and then place the superconductors on them  and place the whole setup in some cold place (like Neptune or anywhere with temperature less than $4K$), would the moving superconductors make it a perpetual motion machine? 
PS: I know perpetual motion does not exist, so please tell me what would stop it from achieving perpetual motion.
 A: One thing that would happen is that the "permanent" magnet, say comprised of any rare earth, would slowly degrade and the magnet would fall.
Recall also that at potentially 'Zero' Kelvin there is still the Heisenberg effect in operation - you cannot have zero momentum - there will either be wiggling due to delta x or delta v - you cannot predict 0x and 0v - the uncertainty principle says something's got to wiggle, even if minisculely, and system energy would gradually degrade.
Why do you thing that twisting the steel strip into a Moebius ring would extend the life of the system?  Do you think the superconducted current flow would cause the magnetic field to flip-flop?  No such luck.  Think of the current flowing down the centerline of the strip - one direction only, and no change in magnetic polarity.  Or maybe you think the twist would have some other effect.  Not one that simple me can come up with.
A: The superconductor would move forever, but you wouldn't be able to extract any energy from the system without slowing it down.
Perpetual motion machines, from which you can extract "free" energy, are impossible. Machines which just move forever, but you can't extract any energy from, or interact with, are possible (however, there would be no way to know if the machine is actually moving, since observing it, e.g. by bouncing a bunch of photons off, would slow it down).
I'm not really sure what the mobius strip is needed for, you could just have a plain round track of magnets.
A: Just because the Moebius strip is floating doesn't negate friction, it just reduces it.  Neptune has an atmosphere, and atmospheric resistance (what we know as "air resistance") will gradually slow the Moebius strip down.  Even in deep space, chance particles will collide with  the strip, which will slow it to a halt after millions/billions of years - not quite perpetual, though practically indistinguishable from our relatively short-term perspective.  
