It doesn't really do anything... But I see it in the output when a player leaves. https://gyazo.com/95c2b57f6ddd37b6aa22ad40547ecab9
This is a very common error usually caused by a core Roblox script (when player leaves, script malfunctions and error appears).
assetgame.roblox.com/game/gameserver.ashx
This is the link to which you're receiving the error from.
The good news is this script should never effect any of your client-side scripts because this is occurring in Roblox's script. It also won't effect much because since the player leaves the game, the script cannot follow up with any more errors.