Anyone who uses a word processor for very long inevitably has the experience of putting hours of work into a long document and then losing it because the computer crashes or the power goes out. Until the moment that it disappears from the screen, the document seems every bit as solid and real as if it had been typed out in ink on paper. But in the next moment, without warning, it is completely and irretrievably gone, as if it had never existed. The user is left with a feeling of disorientation (to say nothing of annoyance) stemming from a kind of metaphor shear--you realize that you've been living and thinking inside of a metaphor that is essentially bogus.
The whole essay is a great read, with lots of insights into fundamental ideas about operating systems (if you think this metaphor is good, wait until you read his description of Windows vs. Mac OS vs. Linux as car dealerships). I think about metaphor shear anytime I'm forced down to the guts of computers or operating systems.