To reproduce the problem:
1. Type in a contenteditable element.
2. Click outside the contenteditable element on a focusable control that does not support text entry (e.g. a button).
3. Type an "x".
The blinking caret is still in the editable element after step 2, even though focus had left it. The blinking caret should have been gone.
After step 3, an "x" was added to the editable box and it had reclaimed focus. Neither of those things should have happened.
I have witnessed the bug on the webkit nightly at SVN r146339. The bug has affected Safari and Chrome for years.
Here is a workaround, dated July 2011:
It's the top web search result for "webkit contenteditable workaround".