Katy Perry performing in front of a leaving crowd at Jingles Ball show

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If that was the last song, this post is a bit tendentious 

This happens all the time in big venues for concerts, football matches, etc. There's people who want to avoid leaving all at the same time. Last time I witnessed that was when Gaga started singing HMH in CBT. That was quite shocking to see but 100% understandable