This modified Atwood's demonstration starts out with both sides in balance as with a regular Atwood's.  The modifications is that there is 340 grams on one side.  The other side has a 40 gram pulley with a 100 gram mass and a 200 gram mass hanging off each end of the string over it.  Connect the 100 and 200 gram masses together with a string so that they can't move and then balance the whole system.  When you burn the string between the two masses the 100 gram mass accelerates upward and the 200 gram mass accelerates downward.  The unexpected observation is that the 340 gram mass will also accelerate downward when the other two masses start to move, although you will need high speed video to see this clearly.  A video at 200 frames per second works nicely.  A neat demo illustrating accelerated reference frames.