Using Chrome, Firefox, and Safari (all for Mac) when viewing pictures in any article, wiki, or forum thread . If you expand an image and close it, hitting the back button takes you to the last image opened in the page. Multiple steps compound this issue.
E.g. If I'm on the homepage and go to see a story with multiple images, I read the first chunk of text then view the first picture as I scroll past it, escape (or click the X) back to the article from the viewer, and view each image individually, closing each one after viewing it, (rather than all at once using the image viewer) and possibly reading/leaving replies.
When I'm done reading a post, I want to use the back to the front page and read the next article, so I swipe back and it reopens the last image from the viewer. I go back again, and it closes that image. I go back again, and it reopens the previous image, repeat as per necessary. So in the story linked above (as an example) it takes 21 steps (19 if I didn't open the bottom image Gregg uses every week) to get back to the front page.