Not sure where to fit this (it could go equally in Current Affairs, Lifestyle or the Internet forum), so if one of my fellow mods wishes to move it, free free.
The BBC have an interesting article on the dangers of rumours in certain situations at
http://www.bbc.co.uk/news/uk-14490693
Apparently, there were several rumours that spread around various social networks about this week's riots. Anyone of which could have sparked further trouble.
The problem with Social networks (and I am counting forums as a sort of social network) seems to be that a significant number of people take what they read online as the truth. It's not, always. But, they take it as the truth. They then tell their friends (on or off line), and their friends tell their friends, and so on. So, one person can very easily say something that is incorrect (whether by mistake or deliberately), and very quickly, hundreds if not thousands will think that what they said is correct.
Take, for example, the Internet Explorer users are stupid story. Someone set up a website that purported to have evidence that they are. Someone passwed the URL to a couple of news media companies, and all of a sudden, at the very least The Register and the BBC are telling tens (if not hundreds) of thousands of readers that Internet Explorer users are thick, and people believe them because they are both respected news sources.
Or take my own experiences. If you believe Twitter, my local shopping centre was absolutely devastated by the riots on Monday. Primark alone was blown up at least 7 times over the space of 5 hours, and Top Shop/Top Man was blown up 3 times. When a journalist from the New Statesman wrote an article saying they were wrong (and providing photos as proof), several twitter users threatened him with legal action.
While both examples I gave above actually happened, they aren't that serious. They could have been though. If someone passed a rumour around that someone had died at the hands of the Police, that could have sparked another riot, and caused death(s).
The article does give the advice that you should attempt to filter out the rumours from the truth, and we were trying to get people to do this in the riot thread, but when you are told something 40 times in the space of 5 minutes, it can be difficult to determine whether it's correct or not.