One onstage moment made Katy Perry roar with pain.

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As the “Firework” singer continues the Canadian leg of her Lifetimes Tour, she shared a look back at her life lately, including a mishap from one of her concerts that left her with a bloody knee.

In her Aug. 6 Instagram roundup, Perry posted one snap of a massive, bloodied gash on her right kneecap as her legs rested on the rim of a bathtub. Around her knee, other bruises appeared in a circular pattern.

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“It was a slide on my knees mome that usually makes them go ahhh,” the “Teenage Dream” singer texted her friend in the next slide, “and it just made me

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