"Then theres Johanna Mason...who won a few years back by pretending she was a weakling."
-Katniss describing on how she won her Games in Catching Fire.
The 71st Hunger Games were won by the 17 year old Johanna Mason. These Games were quite popular, since the way she won them were very interesting.
Pre-Games
Since she mentioned on how the tributes of District 7 wore costumes resembling trees for 40 years during Catching Fire, it can be safely assumed that she wore this outfit too.
Since she wanted to act weak and helpless during the Games, she sold her act by getting a 3 for her training score. Since her score was so bad, her odds of winning were 25 - 1. This year, Lysander Cayhen would be the Head Gamemaker this year.
71st Games
During the Games, as mentioned earlier, Johanna very convincingly acted weak and helpless, so that she would be ignored, but when there were only a handful of tributes left, she proved to be a vicious killer.
When the most liked tribute was killed by a trap by the Head Gamemaker, Lysander Cayhen, he is fired for it.