If we drag-n-drop any artwork into Swatches, a new pattern swatch gets created based on this artwork.
But if we drag-n-drop an singular object with an existing pattern applied to it — nothing happens, as if Ai tries to create a nested pattern and refuses to do so. I think it’s quite obvious what a user intends to do in this case... but we get no warning.

When we use Pattern Options panel/mode, and fill an object within a pattern with another pattern, and try to save, Ai throws us a message (rather vague — Ai knows for sure what restricted element we have, but won’t tell us specifically).
Why not throw the same one (but better, with clear 'no nested patterns!')?