Actually, it could be too lean or too rich. When it is too lean, the increased vacuum when the engine speeds up and the throttle snaps shut will make a low pressure lean mixture that won't burn and the engine stumbles or dies.
Same thing with a rich mixture except the increased vacuum pulls more fuel through the idle jet. The increased amount of fuel and the decreased amount of oxygen (high vacuum = low air pressure = less air) also produces a mixture that won't burn.
Another thing to consider, electronic ignition systems don't do well at low engine speeds because the crank position sensor output gets fuzzy. If the idle speed is a little low, the overshoot when the throttle is snapped shut can drop the engine speed below what the ignition system can tollerate.
