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If you select the "Public Terminal" checkbox when logging into your account, then we currently add special "no cache" headers to ensure that the pages returned to the browser are not stored on the local computer, so that even after you logout, someone can't click "Back", or even go back through the browser cache on disk to see what you were reading/doing.
However it was pointed out that when you go to the view message screen, the subject line of the message was being put in the page "title", which would end up in the browser history, though just the subject line, not the message contents. (The page history is usually visible from the popup menu next to the "Back" button in most browsers that lets you go back multiple pages in one go). We've now changed the code so that if you've used the "Public Terminal" checkbox when logging in, then this is no longer done, so there should be no visual information in the browser history or cache on the machine.