If you draw too much noise, they will probably ban your IP.
Usually if you take a look into the TOS, they will explain in detail what you are and are not allowed to do.
Google for instance asks for you to use their API which limits to the amount of pages you can pull per day. If you instead decide to scrap their engine and request 100's of thousands of URL's a day, you will end up banned.
(I believe, if I remember correctly)
1. No system is safe.
2. Aim for the impossible.
3. Have fun in cyberspace and meatspace.
For those that know