How the hell can you sign into an other person's account while staying logged it?!
They fucked up big time.
I never use twitch but what does having a CC on there do?
When I go to the homepage, I'm logged in as ogsZyphent, whom I've never heard of. If I go to any channels, I'm logged in as myself.
This is potentially a huge fucking issue, given that Twitch has my credit card information.
That and read the first page of someone's PM's. This is a major fuckup, but like you said it seems like someone wouldn't be able to do anything too horrible.
It doesn't look like you can actually do any damage. Worst thing that happens is you see the email address of what appears to be only 2 accounts.
Edit: it seems that you can't do anything horrible with other user's accounts. So that's good, at least