Rashard Mendenhall and Reggie Bush are star NFL running backs who have found themselves backpedaling after recent controversial remarks made on Twitter.

Todd Reynolds is a sports agent whose recent comment about same-sex marriage on the microblogging service has sparked a firestorm in the NHL.

Orlando Magic star Dwight Howard is drawing flak for using the up-to-140-character publishing tool to call out the Orlando Sentinel for what he called "dumb articles."

And the Ultimate Fighting Championship just announced a contest encouraging its mixed martial arts fighters, at least in part, to compete for Twitter followers.

In five years, Twitter has become an increasingly valuable tool for communication, powerful enough to help spur uprisings in the Middle East that have toppled governments. But it also has proved that when it's used recklessly, Twitter, which instantly transmits unfiltered tweets, can cripple one's reputation.

Many incidents have resulted in fines, suspensions and discussions about team and league-wide bans of Twitter, with many sports figures hiring social-media experts to avoid mishaps.

"You can start up a business and you can build a brand very quickly" with Twitter, said Gene Grabowski, a senior vice president with Levick Strategic Communications in Washington.

"But the downside is, you can destroy a brand very quickly."

Experts blame a lack of social-media training coupled with Twitter's simple and accessible interface for many of the missteps.

"Some people just don't realize the harm you can cause," said Ellyn Angelotti, a faculty member at the Poynter Institute, a journalism school in Florida, who examines social media and digital trends. "Some of these processes aren't really inherent to people who don't do journalism."

There is also the misconception that short messages are trivial, said Thomas Cooper, author of "Fast Media/Media Fast" and a professor of visual and media arts at Emerson College in Boston.

"The shorter the message," Cooper said, "the easier it is to distribute and the easier it is to understand."

Or, in Mendenhall's case, to misunderstand. The night Osama bin Laden's killing by U.S. forces was announced, the 23-year-old Pittsburgh Steeler tweeted, "What kind of person celebrates death?" while also questioning Al Qaeda's role in the Sept. 11, 2001, terrorist attacks.

Upon seeing the negative reaction, Mendenhall quickly deleted the tweets and wrote a blog post trying to clarify them. But the damage was done, and Champion, an athletic apparel company, dropped him as an endorser.

Bush, the former USC star and current New Orleans Saint, entered into his own fray during the NFL draft by tweeting, "It's been fun New Orleans," moments after his team selected a running back.

Bush dug himself an even deeper hole by tweeting that he enjoyed the break caused by the NFL lockout: "Right about now we would be slaving in 100 degree heat, practicing twice a day, while putting our bodies at risk for nothing."

After much negative reaction, Bush tweeted: "FYI last tweet was a joke!"

Experts say it can be hard to escape regrettable tweets in this digital age, particularly because the Library of Congress has an archive of Twitter since the service launched in July 2006.

But they say brands can be repaired if demands for transparency are met — quickly.

"As soon as it's recognized that you've crossed a line, you retract," said Ralph Cindrich, a sports attorney and agent, who said he advises his clients to a) not drink and tweet; b) ask team leaders if you're not certain; and c) "tweet only what you know."