Episode 43 of "Sabrina, the teenage witch"
s02e19 When Teens Collide

Sunspot activity causes molecular instability for the witches present in the moral realm: Zelda's left arm attaches itself to Hilda, who is auditioning to become part of a classical trio, and a black hole forms in their kitchen sink. Because Sabrina is half mortal, they are not sure how she will be affected. Participating in a charity drive to collect food for the homeless, Sabrina is at first relieved to experience no worse symptoms than temporary hair loss... but when she protests at Libby's conniving with Mr. Kraft to turn the drive into a competition, things take a turn for the worse: Sabrina and Libby bump into each other and exchange personalities. While Libby become sweetness and light, Sabrina becomes a teenage bitch who soon rules the school and - with the connivance of Salem - begins to plot world domination. When she is arrested and brought to trial, it's up to a repentant Salem to find a way to save her.