On Moodle 2.1
In the book module, when adding a chapter, on the first time using the HTML source button to embed an item (item type doesn' t seem to matter), the window loads and the user can embed the code. User saves, all is well.
If the user wants to re-edit the page and subsequently re-edit the code, the HTML source box appears, but nothing else- the user cannot see the code, nor can code be entered. The user can no longer access the code to the page in the book. The user can move, maximize and close the box using the header bar on the box, so the box is not frozen.
We have replicated this on different Moodle installs, and it appears only to happen within the Book module - the HTML source box is acting appropriately on other pages, activities, etc.