Keep a close eye on who you’re following on Twitter. According to this thread over on Hacker News – you can make anyone you like follow you (virtually speaking!) by simply typing “accept” followed by their username into a new tweet. e.g. “accept DaveNaylor” (without the quotes). Not that I would in any way condone you messing up Dave’s “666″ follow count.
(I concede that ‘vulnerability’ might be a bit of a stretch, but it’s not good, is it?)