Missing the target because someone moved it
Some time back, I learned about Fitts's law. It sometimes pops up as a citation on a discussion about computer user interfaces. If there's motion and pointing and targeting involved in the interface (say, in a GUI), there's a good chance it's related.
I think I first heard it used in the context of the Mac interface, and specifically for the top bar which is there no matter where you go. You can be in the Finder, or Firefox, or something else entirely, and there's always a single bar at the top which has a bunch of pull-down menus and other potentially useful things. The pull-downs change depending on which program you're "in" at that moment but they're always in the same place.
The idea as it was related to me is simple enough: it's hard to "miss" the top of the screen. All you have to do is keep pushing "up" with the mouse (or now, a trackpad) and eventually it won't be able to go that way any more. The pointer will reach the top and will be unable to go any higher. Once this happens, you're free to click on something in the menu.
I think of this as a column of infinite height over each of the menu items. Even though the pointer doesn't actually move higher than the topmost row of pixels, you're driving the input device as if it was going for a button which was impossibly tall. Imagine the "File" menu, for instance, but while it might be an inch across, you can "aim" for anywhere at or above that point and you'll still manage to hit it.
Now imagine a button labeled [File] with the same font and all of that, only somewhere else on screen. Instead of being able to do a rough "throw to top and finesse horizontally a bit" type of movement, you now must control your inputs with sufficient precision to hit the exact box. If you go too high, you will miss it (and possibly hit something else).
This is why having clickable things at the very edge of the addressable pointer space makes sense to me. It's also why I can't figure out what Apple is up to these days with their "Notification Center" thing on Mountain Lion.
I've been using Macs for a couple of years now, and as long as I can remember, the top left corner has been the Apple menu, and the top right corner has been a search widget. This basically meant I could "throw" the pointer at the top right corner, then click, and type in something to find arbitrary files on my machines. I could also go back up there to access the results of my last search if needed.
By being both the topmost and rightmost item, it meant I didn't really have to think about it, and didn't have to be careful, either. Using the inertia of my input device would let me hit it easily.
Then, one day, I installed Mountain Lion (OS X 10.8), and this happened:
The "Notification Center" had taken over the top right corner, displacing the search widget. I'd "throw" the pointer up there like always and I'd trigger this stupid thing:
Great, it's a list of janitorial tasks, and other uninteresting things. It's nothing I care about, in other words, and now I have to purposely adjust every time I want to search for something.
What's more, it had a tendency to "pop up" and would try to get my attention about more of these uninteresting things whenever it felt like it. I thought we established that pop-ups and pop-unders and all of those were evil back in the '90s! Why would we purposely have one that's always on?
So, I found how to turn it off and did exactly that.
launchctl remove com.apple.notificationcenterui.agent
It takes a little bit for it to figure out exactly what's happened and get the icons back in the right spots without any weird gaps, but it eventually works out and I'm back to something like this:
It stays away... until the machine is updated, that is. Every time they do an update to OS X 10.8, this stupid thing comes back, and I have to run that command to whack it once again.
I assume this is where things are going: more and more little annoyances which have to be taken out and shot every time they have a chance to be revived by their software updater buddies.
What I still can't figure out is why they gave up some prime target real estate to something which mostly opens by itself whenever it feels like it. Meanwhile, the search tool, which is something you have to access deliberately, is now that much harder to hit.