Comment 2 for bug 390724

No not at all. Default theme, everything set to default. Do not disable text below icons (else you will not have the issue).
Keep everything default and you will see it.
A screenshot might not help much because you won't see the button borders/margins unless you hover over the buttons.

If you want, I could post 2 screenshots, one hovering over the back button and one hovering over any other button (like forward).
But you can check it out easily on your own install. It is 100% reproducible all the time.