It was a subtle mistake, some didn't even notice it, and some thought it was cute.
We are referring to SNSD SeoHyun's special stage with Kim Chang Wan, and we can't blame her if she was a bit nervous during their performance. Who wouldn't be if you are standing there alone, singing your senior's song.
What's admirable was how SeoHyun handled it, she naturally hummed the lyrics which she forgot, went on, and finished with a great performance. After that, she went to Kim Chang Wan during the ending and talked about what happened.
You can witness these moments through the next clip~
You can also watch their full performance here.