Many of my recent posts have been about sites that have not yet enabled IPv6. For those of you who have done so: Good on ya! Now, do you monitor it?
I've been encountering an increasing number of web sites with AAAA records, but the addresses listed therein either do not answer (timeout) or refuse the connection (RST). On dual-stack networks, these conditions are often masked by Happy Eyeballs. These situations could also result in inconsistent behavior.
Many mobile devices are being released with IPv6 only. This means in the case of a timeout, the user see an unreachable message (and in the case of a timeout, wait to see it) because there is no IPv4 network on which to fall back to try the A record. These aren't just fringe hobbyist users. This is the booming mobile market driving the continued growth of the Internet.
Make sure you not only monitor your IPv6 the same way you do IPv4, but also act on it the same way. Treat outages on either network as production. Your customers will thank you.
(Joni adds: Be sure you really are monitoring what you think you are. Some tools, when presented with a hostname having both A and AAAA records, will do standard fallback (eg: curl, dig), whereas some tools require the protocol be explicitly called out (eg: ping / ping6).)