I've got Hockey Night In Canada on in the background. They're doing the national anthems. The singer gets most of the way through O Canada and stops singing, letting the crowd take up the song for a line or two. He then jumps back in and finishes it off.
I've seen this done at so many concerts that it's almost become a cliche. I have never seen this sort of thing done for the national anthem.
I guess I like it, though it seems a bit odd.