When you click on a picture on Wikipedia, a preview page is opened (that can be closed by using the "X" button). The issue is that when you hit the browser's "back" button the preview window opens up again. The expected behavior is that the back button should bring you to the page you were browsing before Wikipedia. This is the default behavior adopted on other websites (e.g. Facebook) and it's the behavior that I normally would expect.
Since this issue has been bugging me, I am starting to get used to *not* use the preview at all (by opening pictures in new tabs), so I'm reporting this because I don't want this useful feature to backfire and become an annoying feature.