History error

[Link]

I am having an issue where, when in a thread, if I click the back key, it remains on the page, but instead adds the page once to my history, so if I click back several times, my history grows each time, rather than going back. I haven't yet been able to find a reliable way to replicate this. However, I do notice that when it happens, the status bar shows it is loading information from the urls of the advertising, which then changes back to "done" without changing the page

Noticed this on 2 seperate machines, both running Firefox 1.5. Will grab some screen shots if it happens again