In a special OVA of Kuroshitsuji, Ciel Phantomhive stumbles into the world of Alice in Wonderland, a story written by Lewis Caroll. The OVA begins with Ciel doing paperwork in his mansion when he notices Sebastian having a white bunny tail and two rabbit ears. Ciel chased after Sebastian who, similar to the White Rabbit, held up his pocket watch stating that he was late. Ciel continued to follow Sebastian down the corridor until he came upon a room and fell into a colorful place filled with many doors.
Source: Anime Sensei on Google+