Bright teal vertical line

A bright teal vertical line has appeared in my photograph, but it extends above and below the canvas. It doesn't seem to be attached to any layer of the photograph - even when I hide all the layers, it's still visible (see 1.gif). When I zoom out completely, it still extends beyond the canvas (see 2.gif). And when I zoom in completely, it does not appear to have any width (see 3.gif). I have tried opening a blank canvas, and it doesn't appear, so I suspect it is part of the photograph file. Does anyone have any idea what it is, and how to remove it?