I heard this on the BBC's Front Row programme, as they were interviewing Steven Moffat, the latest head writer for Doctor Who:
The sonic screwdriver, for example, can do anything you want. Unless it's really vital to the plot, in which case it can't! Then he either forgets it or leaves it in the TARDIS, or there's a deadlock or a triple deadlock or something. He can never get through a door that's really important!So now you know how to write a science fiction story. Real hardware, of course, never behaves like this.