One of my best memories of Rome is of the Pantheon. I had decided to spend my first evening in the city at Piazza Navona, and stumbled across the Pantheon on my walk there. I was surprised because for some reason I thought it was closer to the Roman ruins.
I not only found it by accident, I entered in the middle of a concert of a children's choir from Bangladesh. There were singing Ave Maria when I walked in and I instantly had chills, it was incredibly powerful and it felt so right in this ancient building. The followed up with a gorgeous version of Amazing Grace.
A short time later, I was in the Piazza Navona and the children's group walked passed me - they were out of choir mode and into kid mode, helping them fit right into the crowd at the Piazza.