As of gtk+ 3.6, the documentation for GtkTreeModel "rows-reordered" says:

"iter : a valid GtkTreeIter pointing to the node whose"

The sentence is incomplete (should be "whose children have been reordered"), and not entirely correct as well: if you reorder the top-level elements, such iter can not be valid, because it will have to point above the top-level. 

GtkTreeStore has an internal root element, so the actual iter in this case can be used for some operations (for example iter_next and iter_previous will correctly return false, because root->next and root->prev are always NULL), but other operations crash or emit warnings. If there is a chance that the top-level elements are being reordered, one should always check for path depth == 0 first.