A blank tool window and a blank script error window, but only after clicking on the Description tab in ArcCatalog.
(Someone reported that this error also occurs when you edit the item description (add metadata) for a model or script and appears to also be fixed the same way)
In internet options, accessible through internet explorer or the control panel, go under the Security tab and set the level to Medium. There are no more script errors. To experiment, I went back and set it to Medium-High, the script errors returned, back to Medium they're gone!