Feeding back into a rail of a power source
That is a good way to destroy everything connected between +5 and GND. 

The motor will draw a lot of current (especially on start-up) and that will typically cause the +5V rail to rise up towards +12. 

Most (positive) switching supplies and linear regulators are not designed to sink current from the output, only to source it, so as soon as the motor current exceeds the draw of everything else on the +5V line you'll have disaster. 

Below is part of a schematic of a 200W PC power supply from here:



As you can see if you connect a resistor between +12 and +5 SBD1/D83-004 will become reverse biased and there is nothing keeping the +5V rail from rising other than capacitor leakage (the feedback might switch the supply down in time, if you are lucky).