My wife used poly over shellac on a built in china cabinet she refinished. She was having problems with the shellac clouding up if anything was put on it. Her Cat jumped up on the cabinet and the heat of his feet left tracks you could see. This happened after it was completley dry.
She used the Bulls Eye shellac (formerley called 3# white).
She solved her problem by topcoating with poly.
The nice, warm shellaced finish is gone, but at least the cabinet can be used. Have seen no adhesion problems to date, and the re-finish is about a year old now.
Just tilt your head a little and it will look straight!