I lost bitcoins to a bitcoin doubler website. They were claiming to be giving [Suspicious link removed] to the investors in 3 days and I was having 1btc lying around, so I invested in it.
The next day the website was down and it was never up again.
Even I have lost once in doubler site in the past when I was a newbie and I invested 0.1 btc hoping that it will get doubled in next 24 hours and as expected it never happened.