I was in lobby waiting for my turn to champion select, when the other teams countdown finished before my turn to pick I was booted from the lobby and given a leave timer penalty with the message saying that "I had left".
Yeah sometimes that can happen. I don't think this should affect anything important if it's not frequent. If it's your client that's lagging, I would recommend checking out this link: https://support.riotgames.com/hc/en-us/articles/201752664-Troubleshooting-Connection-Issues , and do what's called a "DNS flush". There's a nifty video that walks you through the process.
You can also get 1 on 1 tech support with a RIOT employee as you go through it; I found that it made MY lagging client much faster, and the same might work for you.
Cheers :) [vlad-salute]
Welcome to shitty client that Riot doesn't care about fixing :)
