A flaw in twitter allowed users to add ANYONE as a follower of their tweets.

Normally, the other person has to initiate such "following."

This is how it is done.

Many people actually went ahead and added a number of celebrities as their followers. The best part is that these celebrities actually did become their followers and received the tweets from people who had fraudulently added them as followers!!

As word of the hack spread, the panic grew increasingly high, escalating when Twitter temporarily reset everyone's followers count to zero.

Ref: http://bits.blogs.nytimes.com/2010/05/10/twitter-software-bug-exploit...
Ref: http://news.yahoo.com/s/ap/20100510/ap_on_hi_te/us_tec_twitter_glitch
Ref: http://news.softpedia.com/news/Twitter-Bug-Allowed-Users-to-Forcefull...