This particular message is usually displayed when you have stayed in a page for long without taking any action. I sometimes experience it too especially when I open a page and then minimize to do another task either in browser or outside the browser.
Simply refresh the page and continue if your internet connection is not interrupted. If it doesn't work, you should try update your browser as suggested by others.
Everyone here has experienced it that we have all gotten use to it and know what to do once it happens or better still how to prevent it from happening. I just say "damn" whenever it happens and hit on the refresh or reload button after which the user can continue to enjoy their business on the forum. But why doesn't this happen on other platforms or forums being powered by the SMF?