Objects in a clipping mask do not respect "Object Selection By Path Only".
- Draw a filled shape.
- Use Draw Inside to draw another filled shape inside it.
- In the 'Selection And Anchor Display' pane of the prefs, ensure that 'Object Selection By Path Only' is checked.
- Click and drag with the Selection or Direct Selection tools in a part of the first object that does not contain the second object. Note that nothing gets selected unless you drag over the first object's path.
- Click and drag with one of those tools in a part of the first object that does contain the second object. Do not cross any of the paths while attempting to drag out this selection area. Note that Illustrator acts as if "Object Selection By Path Only" was off, and either selects the whole clip group (if you're using the Selection tool) or starts dragging the second object (if you're using the Direct Selection tool).
The desired effect is that step 5 should only select any objects whose paths you cross with the selection box, rather than immediately dragging them. This drives me crazy every time it happens.