If you've ever wrangled a user interface, you've probably heard of Fitts' Law. It's pretty simple -- the larger an item is, and the closer it is to your cursor, the easier it is to click on. Kevin Hale put together a great visual summary of Fitts' Law, so rather than over-explain it, I'll refer you there.
This is more like a contrapositive than an opposite. When I hear talk of the âoppositeâ of a logical statement, I think ânegationâ. Anyway, a great point, and excellent examples.
Actually in GMail you CAN undo a sent mail.
And how about the minimize, maximize and close buttons in Windows?
I have clicked âcloseâ many times when I really wanted to maximize. They should not be so close, I think.
Why is there a dog in the jet fighter?
What you say seems to be entirely true.
⌠except that in gmail I never even thought about it, and never had any problems and in wordpress every single time I try to post something on my blog I sit there thinking Ummmmmm⌠for a moment before I work out where the button is⌠Every single time!
I guess that probably says more about me than the application though.
Just like the âRESETâ button in HTML formsâŚ
WHY IS IS THERE?!
Has anyone here ever used it on purpose?
If I donât want to send the form, Iâll just navigate away from the pageâŚ
Actually, I think your WordPress example doesnât support your argument very well. Yes, the two buttons are far enough away from each other that you probably wouldnât accidentally click one when you meant to click the other, but the âMove to Trashâ link is only slightly smaller than the âUpdateâ button, and more importantly, the âMove to Trashâ link is near the left screen edge so is probably easier to click than the âUpdateâ button (according to Fittâs Law).
Also referenced as âWings Fall Offâ by Gary Larson (http://forums.mozillazine.org/viewtopic.php?p=2383772#p2383834) who also has a couple of comics on the importance of interface outputs (http://s2.hubimg.com/u/209665_f520.jpg, and one I canât find that Iâll refer to âDang if it isnât the big oneâ that someone may be able to point to).
(Why canât I hyperlink to my bookshelf, anyway? Reality needs more features! 
Hey Now Jeff,
That is a ruff subject line for an example. Thx 4 the info,
Coding Horror & SO Fan
Catto
Reminds me of that Far Side cartoon that shows a picture of a guy in an airplane seat with the caption âFumbling for the Recline button, Bill unwittingly instigates disaster.â On his armrest is a series of switches, including one next to âReclineâ labeled âWings Stay On/Wings Fall Offâ.
I think most apps Iâve ever maintained have had a prominently-placed âWings Fall Offâ button that was never clicked in testing but is immediately found within five minutes of launch.
Ejector seat lever next to windshield washer? Try Cmd-Q and Cmd-W in Mac OS. Gets me every so often. Alt-F4 â much more sensible. (But both operating systems have window close and minimise/resize buttons next to each other on the window frame.)
Ctrl/Cmd-C and V (Copy and Paste) is also a dangerous pair. Copy clears your clipboard buffer, with no undo.
@Elwin: Regarding the Reset button â we were informed by German clients that the Reset button is a legal requirement on all German websites, for forms that ask for personal information. For data protection reasons apparently. (Bonkers.)
@Jespeterson:âWhy is there a dog in the jet fighter?â
Heâs a dog fighter.
Fittsâ Law is the reason why Apple puts the menu bar for every app on the very top of the screen. If menus are part of the app windows and you have plenty of them open, hitting a menu can be quite tricky. I never thought about that while I was a Windows user, but since using a Mac, I notice the difference each time I sit in front of a Windows/Linux computer. Hitting menus is just so much easier and faster on a Mac, because your mouse stops at the top of the screen where the menu bar is located.
However, even when placing âdangerousâ buttons far aside, they remain dangerous. I think every dangerous button must have a safety grid. That is
either the button has no immediate and permanent consequence. In that case the button can just perform the action, but the app should offer a user to undo the action, if he still pressed the button in mistake.
the button has an immediate or permanent consequence. In that case it should ask the user again. On Mac the typical dialog will have a closing sentence âThis action cannot be undone.â. To not bother the user too much, there is a âDonât bother me againâ-checkbox that you can check⌠at your own risk of course.
Another idea is to put a safety cap over the button. You know, like in the movies 
You cannot press the button, because there is a âcapâ on top of it. You first have to âopen the capâ before you can press the button below it. In case of software, the button would be not directly visible, you first have to press somewhere to make the button appear and then you can press it. A button behind a safety cap needs no safety grid; even if the action cannot be undone, there is no reason to ask the user again, since a user can accidentally click on the wrong button⌠but he will usually not accidentally click twice on the wrong button. Clicking twice can be seen as âYes, I want to do that and YES, I know what Iâm doing! Donât dare to question my intentions.â
With GMail, as with most applications I use regularly, I learn the keyboard shortcuts (âeâ to Archive and â!â to report spam, for example [âmâ to mute the conversation]) so I donât have to miss-click.
For anyone who DOES use the mouse for everything, at least every one Iâve ever observed, doesnât just fly to the button they think is right, they go to the one that is right.
Undo Send is a Gmail Labs feature: http://gmailblog.blogspot.com/2009/03/new-in-labs-undo-send.html . Google Reader also has an ejector seat lever: Mark all as read.
This reminds me of the recent Ubuntu window buttons controversy, where the window buttons were moved to the left side of the window. Many pointed out that it is now easier to close the window accidentally while aiming for the menubar. According to the Opposite of Fittsâ Law, the windowâs Close button should be placed far away from everything else. By the same reasoning, Maximize and Minimize should also not be too close to Close (as Mikel pointed out above).
This is the reason why âCancelâ is a link in wikipedia edit page, while save and show preview is a button.
But I can say that, first time I wanted to cancel an edit, i did fumble around looking for the cancel button. My eyes just glazed over the cancel link as something unimportant.
So, importance is also a criterion in deciding the size and placement.
One more application is that frequently on web sites, I want to find a âdead spotâ I can click on just so I can give the browser tab focus, so I can then do something like type Ctrl-F to start a Find.
With almost everything being clickable, this can be a surprisingly difficult task.
@Mecki: The Mac convention of putting the menu at the top of the screen was indeed more effective when it was first designed. However, it no longer works so well in the world of multiple monitors. I have 3 monitors at work, and getting to the menubar can be quite a journey. (Especially when the cursor snags on the edges of differently-sized screens.)
