Here's and Undo related bug on Internet Explorer. It does not show up on the Mac.
Enter some text. Switch to HTML view. At this point Ctrl-Z does nothing. Modify the text in the HTML view. Then type Ctrl-Z to undo the changes and then type Ctrl-Z one more time. On my machine the whole text will be lost (unless you type Ctrl-Y to redo). This bug shows up in the editor for posting to the forums.
Keep hacking,
Randal..