The short answer is more heat.
The typical response is that it causes more heat. Hoverer the voltage alone doesn’t really cause all that heat. Consider the equation for heat: I^2 R. Notice you don’t find V or Voltage in that equation.
So why would a girl want to increase her voltage? A computer works on binary, basicly a 1 or a 0, On or Off. The on/off switches are so close to one another they can hear each other sometimes.
Normally a girl interested in over clocking her rig, would increase the voltage in order to increase the clarity between the on and off signal. As you over-clock you increase the number of times you flip the switch and it becomes difficult for the computer to tell the difference between an on and off. Most of the heat is generated by the increased through put this is both increased current (I) and resistance (R).
Why not just leave a high voltage: Too high a voltage and the insulation provided by the layer of silicon isn’t enough and you get something like lighting jumping from cloud to cloud or cloud to ground. The insulation breaks down, and you get a short circuit, and more heat.
Interestingly enough, because the circuits are so close to one another, by increasing the voltage, you increase the impedance and do get a slight increase in the resistance, however most of the heat will come from the over clocking.