[Al Skierkiewicz]

Alright,
There has been a lot of discussion so far as to what happens when the motor stalls and why current is so high and much of it is true but...
As with any system there are a lot of variables, but at any one point in time, one of the variables maximizes and takes over above all else. When the motor is turning, the commutator is moving across the brushes such that the load is constantly changing, the windings in which current is flowing is changing and the heat developed is spread across these windings.(even the motors in the servos have three windings.) When the motor is at rest,(i.e. no current flowing) at least two windings are in contact with the brushes so that the motor can get started. (Open one up and look at the motor construction.) When the motor is at stall and the speed controller is supplying full battery to the motor, the load resistance is half the average load, therefore twice the current, (in only two windings instead of averaged over all the windings) and there is some heat developed by a principle called eddy current in the motor armature. Because current is at a maximum, the brushes are getting hot and maybe arcing, and since the motor is not turning, the brushes can't give up their heat to other parts of the commutator. Since the motor is not turning, what little effect the fan had on heat is reduced to zero. As most of you know by now, as heat rises so also do the losses in wire, contacts, etc., adding more heat to the system. So you can see that stalling a motor at full current can cause some rapid heat rises within the motor. The motors we are given are wound with a varnish coated wire. (That's the red color of wire when you look inside.) At a certain temperature, that varnish begins to break down, and even boils off the wire. With no insulation, the wires begin to short together(lower resistance, higher current) and eventually short to the armature of the motor. That burning smell from a bad motor is the varnish and if things got bad enough, the burning lubricant in the bearings.
At other than stall, some of these other variables have more of an effect, like back EMF. BTW it is back EMF that you use to brake. By causing the motor to act as a generator and shorting the output of the motor in "brake mode" on the speed controller, the back EMF is turned into mechanical force that opposes the force of the moving robot.