I used to have a Harry Potter mug, where a picture of Harry at Kings Cross (the one on the Philosopher's Stone cover) appears when you put hot liquid inside. Then it was put in the microwave, and the picture stayed permenantly. I don't know what happened to it afterwards...
I also had a cup, where more liquid goodness would appear the more you drank from it. I wonder where that went as well... I want to go in the loft and find out whether I have any HP stuff up there.