It might ruin your owner's sight and cause them to buy less cat food. Always get in between the monitor and the person operating the computer. For best results, stand as close to the monitor as possible. If you are removed, go and sulk in a corner for a minute, then repeat. Look as innocent as possible.
Keyboards are great to lie down on.
Make yourself as comfortable as possible. Marching over the keyboard several times is fun too. Always chase the mouse. Your owner can't blame you for this, since it's your feline instinct to chase mice. Nothing beats floppies when it's time to sharpen your nails.