This mistake occurs in the second episode of Season 3, called Dead Man's Party. In it, Joyce brings home a tribal mask from the art gallery that brings dead things back to life.

When Buffy comes home from being stood up by Willow, she is wearing a red and purple skirt, a purple sweater, and purple platform shoes. You can see the shoes very clearly when she walks up to her house and meets up with her mom's new friend Pat.

Buffy then goes into the house and speaks to Joyce, who asks her to go into the basement to get the good plates for dinner. When Buffy is in the basement climbing on a step ladder, looking for the dishes, you can see that her shoes are the same color, but now are high heels instead of platforms.