Basically, Scott Adams (creator of the Dilbert comics) had an incident in early 2006 where he accidentally (permanently) deleted a post as well as 500 comments that were attached to it.
Tog identifies a "misleading metaphor" as one of the issues and highlights the importance of using appropriate metaphors when designing software and educating users on how to use it.
Several articles in his site talk about other misleading and confusing metaphors and how they contributed to bugs or problems, and it got me thinking about the use of metaphors a bit more.
Now, I'm a big fan of metaphors - I use them constantly, particularly when I'm talking about anything IT to a "layperson". Communication between "nerds and normal people" is something that is typically not handled well. Frequently the nerd doing the explaining gets frustrated at the user's lack of understanding and the user gets frustrated at the jargon and poor explanations. Metaphors can help, but only if they're used properly. Similarly, when writing software, user-interface metaphors are frequently used. Think of the recycle bin in Windows or the Home button in your browser. These are (usually) effective metaphors.
My sister is fairly heavily involved in AFL. She was talking the other day about how coaches teach young kids the correct techniques for handballing and marking. They tell the kids to imagine the ball as a spaceship and the little valve in the middle of the laces as the spaceman. When you're kicking, the spaceship should be pointing up, and the spaceman should be pointing to where you want to kick it.
The problem with this is that the intended behaviour doesn't match the metaphor terribly well. A spaceship should point where it needs to go, right? In fact, the spaceman should probably point in that direction as well. Essentially, you're telling kids to imagine the ball as a spaceship, but a spaceship that doesn't really mimic the behaviour of a spaceship. It's misleading.
It gets worse though. When they teach the kids to handball, they tell them to hold the spaceship in one hand with the spaceman at the top up and the spaceship pointing in the direction it needs to go. Ok, not bad so far. Then, they tell the kids to imagine there's an icecream in their other hand. To handball correctly, they should smash the icecream into the back of the spaceship. What?
I don't think I need to point out the problems with that one.
This, to me, is a series of very poorly thought-out metaphors. A metaphor should be something that someone can relate to to help them understand the concept in question. The properties and behaviour of the metaphor should closely resemble the model you're trying to present. This is why metaphors like "bookmarks" in browsers and "address books" in email programs work reasonably well, and others like "Clippy" (Word's abandoned instructional paperclip) confused many users.