How about I yell taxi! when I see a skier stack it.
Why would you want to yell out taxi?
I want to see him/her fall on their face.
It's wrong and dangerous to yell at skiers and try to make them fall. You'll make people mad, and you might get in trouble with the ski area staff, too.
Oh come on, you don't know my reputation around here.
It's not about your reputation, it's about being safe and kind to other people. Find another way to prank them if you want to get a reaction out of them, preferably one that is funny to them and yourself and won't hurt anyone physically.