During Saturday's League of Legends World Championships quarter-finals, in a game between Fnatic and Edward Gaming, a bug was uncovered that stopped Gragas from casting an ability.
After Riot stepped in and tried to rectify the problem, the game was eventually remade when a solution couldn't be found. Both teams used Gragas afterwards, and Riot even announced that the chances of the bug recurring were very low.
However, after longer testing, Riot were able to replicate the bug. This sparked the decision to ban Gragas for the remainder of the tournament, along with two other heroes.
“We don’t want to endanger the integrity of the upcoming games or risk additional remakes, so we are are disabling Gragas for the remainder of the tournament," says Riot's statement. "The bug is also reproducible on Lux and Ziggs, and so to be safe, we are disabling them for the remainder of the tournament.”
This change gives tournament players a week until the semi-finals to adjust their strategies.