Every time I'm replying to an email in Gmail's web interface, I always make the same mistake: I click the button on the left.
Why is that a mistake? Well, let's take a look at the other buttons. (Gmail has convenient tooltips to let you know what each button does).
Seems pretty straightforward, right? Each button does something to the email. I want to reply, so—keeping in mind what those buttons have in common—click the button that looks like "reply".
There's a specific term for what Gmail is doing wrong here: it's breaking the user's mental model.
When using an interface, a user constructs their own internal explanation of how the system works. They keep a list in their mind of what actions they can perform, how to perform them and what the consequences will be. That internal explanation is called a mental model. (Wikipedia has a much more comprehensive explanation).
Consider your mental model when driving a car. You expect that when turning the steering wheel clockwise ("right") the car will turn right, whereas when you turn it counterclockwise ("left") the car will turn left. Now consider what might happen if you were driving and the car broke your mental model. You turn the wheel counterclockwise, but the car turns right! At best, you'd be surprised; at worst, grievously injured (or dead).
The consequence of Gmail's misplaced button is less deadly, but the issue is the same. Even more interesting is that being aware of the discrepancy between my mental model and the actual interface does little to mitigate the problem. Even though I know that that button specifically will bring me back to my inbox, my broad categorization of those buttons as "buttons that do things to the message" wins out nine times out of ten. (The tooltip does nothing to help, since I've almost always clicked the button before I have time to read it).
Fortunately, this is a relatively simple user interface problem. I've filed a bug with Google, so with any luck it will strike a chord and the issue will be fixed.