To reproduce, open the attached test case, click near the top to get a caret and type option-delete. WebKit loops endlessly (or until string runs out of memory?) in previousBoundary() in visible_units.cpp.
Created attachment 15321 [details] Test case
<rdar://problem/5304772>
*** This bug has been marked as a duplicate of 14441 ***
5300379 is the new radar.