A few years ago now I used to run a site called RiotStats, and on there I used to compile statistics for players of the game League of Legends. As part of the site I needed to be able to retrieve game logs from clients so that they could be processed on the server. This meant looking at how the logs were stored and created on the game clients. During this work I uncovered what must be undoubtedly either already known to others, and most likely the creators, however I never saw anything so presumed things were just being kept quiet, much like I was doing.
I don't play LoL any more, and I think it's probably long enough know that the problem has been fixed (not that I have checked).
The main problem is that the log files can be manipulated in such a way in that they can reveal information which is time sensitive. For example, who is on the enemy team during the blind pick phase. Now this may not seem like a lot of information, however with a database of players, it becomes a powerful sliver of information.
With each player name known, you could then lookup their play history, and from that give you a good indication of which hero that they may pick. This in normal play, isn't that big of a deal, however in ranked play, could easily get people very... very upset.
I did confirm at the time that the same process worked in ranked play as it did in normal, so the effect is the same. The ability to see the historic picks of players based off data from both RiotStats and LoLBase during pick phase, which ultimately gives you a possible change to counter their picks, or to at least know who you would be playing against.
I have no intent on releasing the code that makes this possible, as I said above, I doubt it even works any more, the LoL developers have certainly had enough time to fix the problem. I suspect that with the wave of log changes that finally killed off RiotStats would have also killed off this. Either way, it's just to show that it was possible for at least 6-8 months.