Actor Ashton Kutcher was a prolific and popular Twitter user. In 2009 he became the first person to have over 1 million followers, beating his closest rival, CNN. But even the most experienced Twitterer will occasionally step in it, and in November 2011 that's exactly what he did.
When Penn State football coach Joe Paterno was fired over revelations of sex abuse in the school's athletic program, Kutcher, who was unaware of the whole story, took to Twitter.
"How do you fire Jo Pa?" he tweeted. "#insult #noclass as a hawkeye fan I find it in poor taste."
After doing himself the favor of finding out what had actually happened, a chastened Kutcher went back to Twitter. "Fully recant previous tweet!"