adoptRef assertion was turned on (bug 41547), but it seemed that we forgot to update RefPtr documentation (http://webkit.org/coding/RefPtr.html).
For example, a few examples contains the following line:
RefPtr<Node> a = new Node;
That would work fine if Node is a reference counted class that does not derive from RefCounted. The new assertion is specific to RefCounted.
I made some changes in <http://trac.webkit.org/changeset/66233> that I think resolve this.