If the SX kill sequence is not used properly, it is possible to get stuck in an infinite loop. To avoid this, place XFK0 as the first command in the XFK sequence. You may then reassign XFK before exiting the XFK sequence.
If you do get stuck in the loop, type BRK to break the loop and set XFK to zero automatically.
Alternately, you can use OFF to break the loop and XFK0 to disable the call to XFK.