This randomly happens and i can not tell exactly what i do or why. All i know is that every time you visit a page,site, picture etc... the back button remembers every page you have just been too in that session. However some times the back button goes grey and i can not click on it and it loses all the previous pages i have been to. I can reopen the last pages by going to my history but it's really annoying as it takes time and as i surf and look at a lot of things, the back button is crucial for me.