When you're experimentally or regularly building and somebody else starts adding to it as you're still working.
For example: I was experimenting with stairs in a large multi-floored rotunda area, and one guy added tile bricks on the tops and bottoms, disregarding the fact that I was standing there taking my time and trying to make geometric sense of this puzzle (stairs are loving hard for me, I would drama stairs if I could). He didn't ask if he could help or even seem to realize that I was actually trying to think.