That "5 X vs. 5 Y" bug we saw pop up earlier was actually an in-development game mode.
A little while ago, we saw some tech-savy League of Legends players discover a "bug" that let every player pick the same champion. Dubbed "5 X vs. 5 Y" mode by the community, it turns out that it wasn't a bug, but an in-development game mode that Riot has now unveiled as "One For All".
One For All will be part of something called "featured gameplay modes" - a rotating suite of wacky and experimental game modes, much akin to Left 4 Dead 2's "mutations".
Featured Gameplay Modes are about ready to burst onto the League! We have a variety of experimental modes - each with a unique spin on the classic League of Legends formula - that we're looking to introduce one at a time. We're limiting the availability of each mode so that you can try out a variety of different game types. After they're brought down, we'll check out your feedback as we develop new game modes - there's always a chance that we'll bring back the ones you love the most, too.
While the One For All mode will only be featured in the Summoner's Rift matchmaking queue, Riot says that players will be free to try it out in any of the game's map via custom games.
Riot did not give a time frame as to when we would see the featured gameplay modes feature released, saying that this is just a very early look at the system (most likely spurred on by the "bug" from earlier in the month).
Source: Riot Games