Most expensive thing a pet or child has broken in your house. Go.
Joey used to ruin every sweater I brought home, and Claire likes to chew stuff, but the prize has to go to Riot, who climbed up onto a windowsill we were sure there was NO WAY he could reach, and knocked a clock we'd gotten as a wedding gift off onto the floor.
The worst part was how totally satisfied he looked afterwards. He was so proud of himself. "Look, guys! I did it!"