QGIS » QGIS Application

Select a polygon layer and toggle to edition
You can activate its vertices by clicking inside a polygon with the node tool. No need to precisely click an edge or vertex.
Now move or reshape the feature.
If you click with the node tool inside the feature (at a place the feature didn't cover before), you get a warning that QGIS couldn't snap to a feature. The only way to activate the vertices is to click on an edge, a vertex or inside the old extent of the feature (though there's no more feature there).
It looks like until you save the changes, the node tool doesn't acknowledge the change of feature extent (just edges and vertices are recognized).