What does it mean that boundary cases test your categories?

Items that do not fit neatly into any category expose weaknesses in your system.

Example: A tomato is botanically a fruit but culinarily a vegetable. A virus is neither alive nor dead by conventional definitions. A freelancer is neither employee nor business owner in most tax codes. When you encounter something that doesn't fit your categories, you haven't found a defective item — you've found a defective boundary. The item is data. The discomfort is signal.

Try this: Pick one category system you use regularly — your task labels, your filing structure, your mental model of your team's roles, or your definition of 'done.' Find three items that don't fit cleanly into any single category. For each, write down: (1) which categories it partially belongs to, (2) what property makes it resist clean classification, and (3) whether the problem is the item or the boundary. If you can't find three misfits, your system is probably too coarse to be useful.