A more fun instance of this sort of thing on HN is when I suggested that HN might be attackable because of a flaw in random number generation and then someone else who hadn't seen my suggestion went ahead and did it.

Me mentioning it: http://news.ycombinator.com/item?id=596126

The attack: http://news.ycombinator.com/item?id=639976

That was an absolutely amazing hack