The Medrash tells us that Esther was the wife of Mordechai, and once she was forceably taken to be the queen and Achashveirosh's wife, she was still permitted to Mordechai because she was taken Beones. Once she went to Achashveirosh to invite him to her first meal, she was no longer permitted to Mordechai because she went willingly. (this being the Midrash's explanation of "Ka'asher Avaditi, Avaditi"(1))
It is perplexing that she was still permitted to Mordechai at that point, because earlier she willingly saved the life of the man who was raping her, by telling Achashveirosh about Bigson and Seresh. At that point she should be considered "willing" and prohibited to Mordechai?
(1) Gemara Megila 15a: R`Abba said: It will not be [she said] according to the custom of every other day. Till now [I have associated with Ahasuerus] under compulsion, but now I will do so of my own will. And if I perish, I perish. As I am lost to my father's house so I shall be lost to thee.