If it causes you to topple over because it ruins your center of gravity. If when you drop the pipe your heart skips a beat not because the pipe might get damaged but you don't want to break a foot or two. If you have to make sure the cat isn't napping in the chamber before you light up. If it's too large to wear as a hat. It needs wheels like those suitcases you see at the airport. If you can't decide to just dump the dottle or try to light the last half but are afraid of falling in. If you just drilled a couple of holes into the earths core. And pretty much any pipe even the Kiseru if you're the size of an ant.