Working long hours these days means visiting lots of websites, which means any browser that's been in use for any length of time will have multiple tabs open. That means any computer user runs the risk of accidentally clicking on the wrong x symbol and, instead of closing a useless link, closing a link vital to the day's work.
However, all is not lost if that should happen. Restore the lost site by clicking the Ctrl (Cmd on Macs), Shift and T keys simultaneously. The trick works in all mainstream browsers, and opens the last closed tab. Doing it again opens the second-to-last closed tab.