Ketaki Flower

Pandanus odorifer.

The blossom that lied for Brahma when he was in competition with Vishnu to find the beginning and the end of the Jyotirlinga, Shiva’s appearance as the primordial lightning. Since then, the flower may no longer be offered to Shiva.

Another flower with which Shiva may not be worshiped is the Champaka Flower.

Read the story here
http://hindumythologyforgennext.blogspot.com/2012/03/why-shiva-is-worshiped-as-linga.html