Motion sensing light triggered by fan on the same circuit
The motion sensor's circuits are probably being confused by a voltage spike caused by an inductive kick from the fan.  The voltage levels in the device are disturbed, reversing some voltage comparison so that the light is activated. The problem can be attacked at either end.  Containing the surge at or near the fan, or preventing its entry into the motion detector.

http://en.wikipedia.org/wiki/Snubber

Perhaps the fan has no snubber mechanism in place to mitigate inductive kicks when the motor is switched off. Without schematics for the fan and its control circuitry, it's impossible to give a specific recommendation such as "get such and such type of diode and solder it across such and such points".  

A suitable film capacitor connected across the power terminals of the fan maybe able to contain the spike, and would make for a very cheap fix. The capacitance doesn't have to be very large (I'm guessing, on the order of .0nnn microfarads), but the capacitor should have a decent voltage rating, of at least a few hundred volts (i.e. beyond the line voltage) so it can take the spike.

http://en.wikipedia.org/wiki/Film_capacitor

The small capacitance of the surge cap appears to be nearly an open circuit to 60 Hz power line AC (a very high impedance). But a voltage spike looks like high frequency content which passes through the cap. The cap will "take the edge off" the spike, so to speak. It just has to do that well enough so that the motion sensor isn't falsely triggered.