First, it shouldn't be allowed (per HowToTestUI) but Piwik allows you to re-add a Widget that's already on the Dashboard.
On the widget menu, click on the widget name (even though it's greyed out and a tooltip says it's already on the dashboard). Piwik then adds an empty widget box to the Dashboard with the title, "Widget not found".
I'm opening a separate ticket as this isn't a regression, but I hope to fix this in ticket #243.
Consolidate in ticket #243 where It has been fixed. (see patch)