For example, let's say a warning pops up that says you've got an airplane at your 3:00 on a collision course with you. And you really don't. So, crisis averted, right? Wrong! The pilot could try and pull some quick hero move and get into an even more dangerous situation. It distracts him from good airplane flying. He trusts the traffic system less once he realizes it's not always right - so the next warning he might not take as seriously but it might be real.
In other words it's very bad.
In life it's usually less bad. I saw a stapler sitting on a table and I had papers to staple but when I went to use it, I stapled it and nothing came out, it was out of staples. Hazardously misleading stapler! Wasted my time, sitting there and not being able to deliver, so if we don't have staples at least hide it away (or, as in airplanes, disable and placard!)
And I totally thought about it this week with the stories going around about facebook's "other other" message box -
There Is An Even More Hidden Facebook Messages Folder
Most people are angry that facebook shoves messages back into this "hidden" folder, behind the "message requests" even.
I got burned by this - a really interesting request by a special company wanting me to taste test some cookies, for real, I never got the message. Don't get me started.
What really makes me mad is the fact that you can look up any Tom, Dick or Harry on facebook, and on their profile page there will be a "message" button, just like all your friends! You click it, you type the message, you think the person will get your message, you're never told that since you're not friends your messages get pushed down into an unreachable basement. They never respond to you and you think what a jerk, after I reached out so kindly.
It's misleading! Either don't put the button there, or warn people how it works, so they keep searching for real ways to contact you!
Usability, people. Think about it.