I'm not sure how best to describe this one. I thought surely it had been reported, but I don't see it listed, so here goes. This behavior existed in both SP 11 and SP 12 v 588.
Some of the sites I commonly visit, such as
http://www.signonsandiego.com/ (San Diego Union-Tribune),
http://seattletimes.nwsource.com/html/home/ (Seattle Times), and
http://www.thestranger.com/seattle/Home (the Stranger) all spawn multiple instances of a single page in the back button. So if I visit any of those sites and click the back button, it will do nothing - because the previous link shows as the same page. I have to click on the little down arrow to get the full back sites menu, and then scroll down (sometimes as many as 30 lines!) to find the previous link.
SP 12 does not like the Union Tribune site, by the way - something about it can crash OW on occasion. It is not reproducible, because most of the time it loads fine. That site drives me nuts; it uses a ton of java and flash for advertising, but you can't turn off the javascript because the main links to get in to read the news are javascripted. Stinkers.