The super-web-app Facebook (which I love and will continue to love even because of this UI oversight) recently had a good design ‘realignment’. Improvements were mainly to be found in menus, quite rightly.
However one little thing I noticed the other day was this poorly thought-out message sending form. Can you guess where the problem is?
If you haven’t already guessed, the problem is that the “Send” button is smaller than the “Back to Inbox” button.
Actually, I think that the real problem is that they’re listed together. I’ve written a message out twice, consecutively, and still clicked “Send to Inbox”. I’ve also clicked “Send to Inbox” without realising what it actually did (assuming, as a user does, that it will send the message), and thought that I’d sent a message.
It’s a little thing, but it can significantly disturb the use of messaging on Facebook.
I still like it, though! 
What can be learnt from this? Take care when placing buttons which significantly different consequences together.
This is also a semantic error, “Back to inbox” should be a link and not a button, they’re just making more work for themselves using a button.
This used to be a common problem back in the day (i.e for me around 1999), where there seemed to be a fashion for having a “clear” button next to the submit button on most forms, which would clear the contents of the form (not sure why - some data entry software throwback maybe?). There was nothing more frustrating than filling out a huge form then accidently hitting clear and having to start again!
This sort of usability issue drives me crazy on http://www.thetrainline.co.uk - ‘Log-in to your account’ is a small text link with ‘Forgotten password’ and ‘Change password’ right underneath as huge buttons and my first instinct is always to click on the latter two.
Hi how are you today im going to get this fone next year its so cool and awesome