For a while now I've been using two metaphors that seem to resonate with tapping clients and students. One is what I have seen referred to as "the spinning wheel of death." If you use a Mac, it's that multicolored little pie icon that spins eternally when the computer is stuck somewhere just short of frozen. That point in a process where you've clicked some button and the computer goes to work on whatever command you gave it....and the spinning wheel indicates "okay, just give me a moment here" and that moment turns into... an eternity. Your computer is, for all intents and purposes, unusable until you perform a hard reboot by turning it off and back on.
For people without Macs (though I am sure there is a PC equivalent), the metaphor I use involves an anaconda snake trying to eat a cow and getting stuck with the cow half swallowed. Without manual intervention, that cow isn't coming back out and yet the snake isn't big enough to finish the job of swallowing the cow! As with the tech analogy, everything stops. Forward progress is impossible and there's no elegant way to back up.
What both these examples have in common is they illustrate a process being interrupted by something TOO BIG. Something that the process can't cope with. The Mac had too many programs running simultaneously and the snake's ability to unhinge its jaw exceeded its throat's potential to expand. Result: Stuck.