Like most people, I spend more time in my Gmail Inbox than I do in my Spam folder. But when I do check out my spam folder, I always make the mistake of marking a message as 'not spam' instead of deleting it because of the button placement. In the inbox folder, the 'delete' button is to the right of the 'mark as spam' button. But in the spam folder, the 'delete forever' is to the left of the 'not spam' button. Is this an example of bad UI/UX?
closed as not a real question by JohnGB, Charles Wesley, Matt Obee, Benny Skogberg, msanford Apr 5 at 2:43
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As Jef Raskin informs us, habits are actually a good thing when it comes to UI, and capitalizing on them is the sign of a good UI.
As Raskin also points out, modes are bad and should be avoided wherever possible.
This is an excellent example of both of these concepts and how they work (or rather, don't) together. By changing the buttons on the toolbar when the user switches to the Spam folder, Google has introduced a mode. Modes conflict directly with habits as you've eloquently captured. Your habit is to click on the toolbar in the location of the delete button, where it usually is. The problem is, because of the mode, that button is not in the same place your habitual behavior expects it to be in.
So to answer your question, yes this is a poor UI as it breaks the habit you've formed in order to use the app more efficiently. The solution is to either introduce the Spam folder-specific options in a way that doesn't introduce a mode, or at the very least put the buttons that are the same between the modes in the same location in each mode.