Ask HN: What should I know regarding Scraping Websites?

Sometimes crowd-sourcing takes time & if you already have a market you can get a lot of data for free via scraping.

So what should I know regarding Web Scraping?

Not all websites provide API, so many people scrape content.

What are the legal implications?

Have you scraped any? If so, anything you've learned the hard way?

Also, is there anyway to prevent website from scraping content?