Bitcoins can be easily used to scam people out of their money due to the inability to perform charge-backs. What are the most notable cases of Bitcoin scams that are known?
Here's a list of the most notable scams, thefts, and hacks: https://bitcointalk.org/index.php?topic=83794
Here all famous scammers/sites:
If you look at a graph of the value of 1 bitcoin, that huge spike and drop was caused by someone hacking Mt.Gox and selling everyone's bitcoins to him/her for $0.00, dropping down the value to $0, and it stayed low as more people sold their bitcoins to make sure it wouldn't happen to them. It's been slowly rising since then.