When page cache is active, then cart page gets cached. If a visitor adds a product in a cart and then when the page is cached, if you visit the cart page as another visitor, you will that product in the cart.
I have tried to exclude the cart URL from page cache, but that's not a solution. As -
1. Floating cart will cart show different information in different pages
2. I have to manually add cart and checkout page slugs for all subsites