It was "fun" discovering this the hard way a number of years ago when active US Android user count for a game we were supporting dropped 15% essentially overnight. The TCP stack in the client only did IPv4.

The challenge, ironically, was convincing management that adding IPv6 was the thing worth trying. After almost a week of getting nowhere (and almost 2 weeks of outage), I forced the issue by saying "Look, I'm doing this. I need one engineer for 2 days. If it doesn't work, then it doesn't work."

He got the change implemented in 2 hours. QA OKed it the next day. The topic never came up again.