I'm not that protective of my piano, I am way more obsessed with keeping everything off my violin. It just seems much more delicate. But nonetheless, there are things that may never touch my piano:
1. Kids with grimy fingers
2. Adults with grimy fingers
3. Chocolate
4. Nail polish
5. Anything else grimy
6. Animals
7. Sharpened pencils
8. Uncapped Pens
9. Toes
10. People who don't actually know how to play piano. Chopsticks does not count as 'knowing how to play piano.' My parents are exempt from this rule.
But people who know how to play, but don't play well are allowed on the grand... depending on my mood. I used to teach a little girl, so she automatically got access to my grand, but on bad days, I prefer those people to stick to the upright.
I did once get blood all over my piano teacher's piano. It was winter and it was very dry outside (it's an icy desert here, with altitude at 3000ft) and my skin cracks a lot. Halfway through playing this one piece, my thumb started bleeding, and it wasn't like I could just stop in the middle and ask for a band-aid....She wanted to take a picture of it to show everyone how she made her students play so much that they bled all over the keys. Mind you, my piano teacher doesn't take that good care of her piano... I think she gets a new one every few years. She has broken 2 strings (don't ask how...) and drops pencils, pens, markers, hi-lighters, paperclips, paper, and other small objects in there. Sometimes, she notices and pulls them out, but other times, you start playing and one key or another buzzes and sticks... Oh well.