Ugh, yes, such a pain. I hate when people walk away from the game during champ select. The champ select time is for SELECTING A CHAMPION. Go figure. XD If you want time to do stuff away from the game, do it before you start a game, or at the loading screen, when you don't have a responsibility to the team to actively participate. If you're going to commit to a team game, be present for the team, please. AFKing is just as bad if not worse during champ select as during the game itself.
What's also annoying is when people lock in an ambiguous pick and don't call a position. Is that Lux going support or mid? massive shrug
And then there are the people who see someone else call mid and say "I got it", and it's like...when was this? If you come in saying "I got mid, y'all", and there's nobody else in the room yet, no, you didn't call mid. And then people compensate for this by copy-paste spamming their position in the chat six times, just to be absolutely sure everyone sees they called it before anyone else had a chance to type a word. If it's that important to you, please use team builder. That's what it's for. Bickering over who has dibs on what position is just obnoxious.