The moment it happensYou ship GET /pets. Someone asks to filter by status, so you add ?status=available. Then by species. Then someone wants cats or dogs, and a query string has no way to say “or”, so you invent something — ?species=cat,dog — and quietly decide that commas mean OR today. Then someone wants pets born after 2020, excluding the adopted ones, and you find yourself designing a grammar inside a query parameter.