My ISP broke their IPv4 gateway one weekend. Fortunately they have their IPv6 stuff working in parallel. I was surprised that only maybe half of the Internet is reachable via IPv6, and there was no pattern which sites worked and which didn't. There's still some work to be done before IPv6 is usable.
One Windows PC on our LAN got flipped into "internet sharing" mode, causing a DHCP battle which randomly killed IPv4 connectivity. That one was weird to diagnose at first since random sites still worked (ones with IPv6 support)
