Can be reproduced with any sample which contains documents and toolwindows, for example the first Simle-IDE sample:
- dock "Solution Explorer" as document
- start drag-float operation with now documentified Solution Explorer
- hit ESC for cancelling the operation
The solution explorer is now docked as tool window again instead of being a document as it was the state before the drag-float operation started.
This also happens with ToolWindows which have been documentified from the very start (defined in XAML) and never ever had been docked before.