This is in the Windows port of the GIMP distributed from the page by Jernej
Simoncic linked to on gimp.org's Windows page.

When editing an image with multiple layers (some of which are smaller than the
canvas size), the currently selected layer has a box drawn around its border.
However, when using the layer dialog box to change the selected layer, the box
is not removed from the previously selected layer. Thus, there is a selection
box drawn around the original layer that was selected as well as the new
currently selected layer. The box that is drawn around the selected layer should
be removed when that layer becomes unselected. This worked fine in older
versions of the GIMP, but is faulty in GIMP2.