Two Clash of Clans accounts on one device
I just developed this problem this afternoon, too.  I have 4 Clash of Clans accounts and I freely toggle between them on my various i0s devices: two ipads and an iPhone 5s.  That's worked like a charm for two years now.  I just keep logging in and out with different Gamecenter logins to load and play each village.

But as of today, I log into GameCenter, load CoC and it appears to work and says "Welcome Back, so-and-so", but then the popup to CONFIRM and load the village I just logged into never comes up.  My screen is stuck on one village and I can't get off it no matter which account I log into Gamecenter.  All of the devices now show just one of the four account game screens and the Gamecenter logins have no effect beyond the welcome back message.  The problem isn't just on one device, but all of them.  It seems to be tied to one of the 4 accounts.

I sure hope there's a way to fix this (like Gamecenter getting their crap together) considering I've been playing Clash of Clans for over 2 years now and my villages are highly developed.