Why does edge cases fail?

The most common reason edge cases fails: Treating boundary cases as exceptions to ignore rather than evidence to examine. The instinct is to force the ambiguous item into the nearest category and move on — filing the tomato under 'vegetable' and forgetting about it. This preserves the illusion that your system is complete while guaranteeing it will fail silently whenever the next boundary case arrives. The opposite failure is equally dangerous: treating every boundary case as proof that categories are useless, and abandoning classification entirely.

The fix: 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.

The underlying principle is straightforward: Items that do not fit neatly into any category expose weaknesses in your system.