In the first chapter of sefer Ezra we read that Koresh sent all the utensils that had been stolen by Nevuchadnetzar back to Yerushalayim as part of the rebuilding of the Beit Hamikdash. However the gemarah megillah 12a based on the pasuk in Esther 1:7 seems to indicate that the utensils that Achashveirosh used at his party were those of the Beit Hamikdash. If that is so, how did Achashveirosh have those keilim in his possession? Were they taken back from Yerushalayim? Is there any commentator who explains this?
In the Artscroll Divrei HaYamim bet page 473 (as well as Artscroll Sefer Ezra ad loc) he cites the Ramban on Megillah 11b who indicates that not all of the keilim were returned during the Koresh proclamation (contrary to Rashi's reading of Ezra 1:11).
I once heard this question asked at an Arachim function. The answer given was that although Koresh had the utensils returned, not all of them made it back and therefore Achashveirosh used the ones that he still had. I have not seen a written source for this though.
Rabbi Hayyim Angel in a lecture on Megillat Esther posits that the gemarah should not be taken literally. Rather, like the gemarah (ibid) which mentions Achashveirosh wearing the clothing of the kohen gadol, this gemarah is alluding to a theme that relates to Achashveirosh being portrayed as eclipsing God and Shushan as the 'new' Jerusalem