What's the worst a debt collector can do?

So, let me ask you, what's really the worst-case scenario when dealing with a debt collector? Can they just show up at your doorstep unannounced? Will they harass you constantly over the phone? Could they even take legal action against you? I'm curious to know the full extent of their power and the potential consequences if you fall behind on your payments. Is there anything we should be aware of to protect ourselves from the worst that they might do?