Accidentally sending out the same twitter tweet 3 times.
Also a really sweet (sweet meaning great) tweet.
To my knowledge, Smashing Magazine gets credit for coining the term when they accidentally sent a tweet 3 times, and used the phrase in an apology.
.@twitteridname The triple-tweet was an honest mistake. Sorry about that, guys.
.@twitteridname Thanks for the twit, that was triple-tweet!