Seems to happen when making > 5 Automation.executeJavaScript calls.
<rdar://problem/26287306>
Maybe it is a frame? It gets injected for each frame as needed.
Created attachment 278994 [details] Proposed Fix
Comment on attachment 278994 [details] Proposed Fix Clearing flags on attachment: 278994 Committed r200950: <http://trac.webkit.org/changeset/200950>
All reviewed patches have been landed. Closing bug.