There's a possible reference cycle between Document and its form elements if the Document gets detached from its frame before the timer started in didAssociateFormControls() fires.
Created attachment 307384 [details]
Comment on attachment 307384 [details]
Clearing flags on attachment: 307384
Committed r215465: <http://trac.webkit.org/changeset/215465>
All reviewed patches have been landed. Closing bug.