So here's a quit significant bug I've noticed with the new champ select: When having played a couple games with different mastery pages, the name of the selected mastery page is sometimes completely wrong. It shows a name of a mastery page that might not be the selected one. You can see this when you try to edit the mastery page, and you'll end up on a different page than the name would suggest.
This causes annoying situations in game too. You'll end up with a singed with windspeakers blessing, and adc with grasp of the undying etc.
It does not always happen, but when it does, it's really really bad.
Hope riot can look into this.