That can happen if a modal window pops up and it wasn't set to correctly have LW as its owner. There's a few stray dialogs like that in the code that we haven't properly caught. You can generally fix it by minimizing your apps one-by-one until LW is left, and the modal dialog will be left in the front. Let us know which dialog it was and we'll fix it.

BTW, for future beta bugs please see the "Beta Bug Report Info" thread.