In 'Calling and Not Calling My Ex' by Okkervil River, the jingly-xmas bells come in on the line where Will Sheff sings "She was so sweet on Xmas Eve".

In 'Amateur' by Nada Surf, the drums change to a kick/snare/hats disco pattern right on the word 'disco' in the line "We hung a disco ball up".

I've just realised that I love it when things like this happen in songs. I can't think of any other examples right now though.