The ending sequence music.

The ending music changes depending on the characters in the episode or at least it seems to, the first ep's where its just yukino the ending music is just one female voice singing whereas when arima comes into it a male voice joins, and also when the ep isnt really based around arima or yukino (ie the tsubasa sub-plot) its only instrumental with no vocals at all.

I just thought this was a neat little extra thing. ^_^