From page 61 of the "Crossfire Specialist" PDF.


The transmitter output signal includes a rolling code that changes in an indecipherable manner every time a button on the transmitter is presses. The receiver is synchronized with a valid transmitter to accept signals with a new code, but not a previous code. However, if the transmitter unlock button is pressed repeatedly while out of range of the receiver, synchronization may have to be restored.


SO, to sum up, don't hammer away on your keyfob buttons!