Steps to reproduce.
1. Open the attached URL.
2. Place caret/cursor in the second editable paragraph.
3. Press the Format block link.
4. Observe the strange span that got added to the start of block element.
Confirmed with WebKit 525.1+ on Mac OS X 10.5. Thanks for the simple test case!
I have a fix for this out for review.
Fixed in http://trac.webkit.org/projects/webkit/changeset/32497
*** Bug 21505 has been marked as a duplicate of this bug. ***