What caused the Aurelion Sol bug that broke Game 1 of TSM-CLG

Update, 9/30: Another Aurelion Sol bug, this time at Worlds, caused the champion to be disabled again.

Sunday, an Aurelion Sol bug caused a remake of Game 1 of the NA LCS semifinal between Team SoloMid and Counter Logic Gaming. TSM would eventually win the series in a 3-0 sweep, advancing to the finals. Monday, we finally got more information as what exactly happened, via Riot Penrif’s post on the boards.

First you need a little background on Aurelion Sol’s passive, which rotates 3 damage-dealing stars around him at all times. It’s a unique ability in two ways:

1. The stars are the “longest lasting missiles in League’s history by quite a large margin,” per Penrif.
2. They are also the first missiles whose movement is defined by another unit (Aurelion Sol, since the stars go where he goes) and does not have the same visibility status as that unit.

This means that if Aurelion Sol is in a bush but his rotating stars extend outside the bush, the player should see the visible stars but not Aurelion Sol.

To deal with this, Riot created a system where the game is sent information about where Aurelion Sol is if his stars are in vision while he isn’t, to accurately represent the stars’ movement. But there is a “small time gap between the client gaining visibility of the missile and when it receives this positional information,” per Penrif, and that’s what caused the bug on Sunday.

Here’s Penrif’s timeline of events. I’ve slightly edited it to include more information from later in his post about what occurred at the 11:06 mark.

A brief timeline, based on Twitch's VOD:

- 10:29 game time (43:38 video) - Aurelion Sol's passive spotted over wall by ward at blue buff

- 10:31 game time - Aurelion Sol and his passive fully leave TSM's vision

- Aurelion Sol remains out of TSM's vision until:

- 11:06 game time - Aurelion Sol's passive spotted in mid lane by TSM Svenskeren and at 10:31 position by TSM Bjergsen (for a few frames)

Basically, as Aurelion Sol returned to vision, the client received the information that his stars should be visible before the information of where the champion was. That caused Bjergsen to see the stars for a few frames at their old position when they were actually elsewhere.

Penrif said the issue had existed since the champion was released in March, and that the code designed to limit its effects “were clearly not enough.” A fix is coming as a part of the 6.18 patch cycle, the one Worlds will be played on.