Maybe this is not such a big danger as those described by the previous posters, but I always felt cautious about the internet.
While it's a great tool that contains a wealth of information, it also makes it easy for anyone to write whatever they want, whether it's truth or lies, and manipulate others on a global scale.
People believe in a written word more than in a spoken word, and people fall for hoax websites. For example, man pregnancy com (NOT real!) or google "victorian robots" - webpage about robots built in the Victorian era (NOT real!)
Or, take, for example, the "deadly" swine flu from a couple of years ago. I strongly believe that we were simply brainwashed and the swine flu was not as dangerous as any other regular flu and was just a campaign to sell vaccines. Maybe not so, but the internet made it easy to reach a large audience.
In any case, it's already hard to separate the wheat from the chaff, and soon it may become simply impossible!