Please Welcome to the Stage: Every Single Surprise Taylor Swift Guest

Who?! Come on, we all know this woman from her iconic role on Law & Order: SVU.
Where?! Mariska appeared on stage in Philadelphia, PA.
Why?! Well, Mariska was definitely one of the many stars in Taylor's "Bad Blood" video. Even more importantly, Taylor named her cat after Mariska's SVU character (Olivia Benson). She's earned her keep.