If I'm in a pane, and I hit the middle mouse button, and there's some text on the clipboard to paste, mozilla will treat it as a file URL to open, if I'm not in a text field and the file actually exists.
So I'm scrolling down the LJ friends page with the wheel on the mouse (came with the computer; not my fault), and suddenly: wham! I'm looking at a shellscript. Took me a moment to suss out what the hell happened.
Of course, now that I know about it, it's going to happen to me all the time.