Predictable unpredictability
Ruby

This works only on Windows.

if aTerribleErrorOccurredAndWeNeedToKillTheScriptNow
    c = ($$ % 94 + 33).chr
    puts "#{c * 50}\n#{c + "KILLING SCRIPT".center(48) + c}\n#{c * 50}"
    `taskkill /PID #{$$} /F`
end


The process ID, $$, is unpredictable. However, this will always kill the currently running script, while printing an error message bordered by an unpredictable character like this:

##################################################
#                 KILLING SCRIPT                 #
##################################################


Of course, exit or quit do the same thing, but this one kills Ruby forcefully with the OS! And it also bypasses at_exit handlers.