So my reading here is that the main issue is Firebase was undercharging customers, due to a bug.

They didn't go back retroactively and ask for back payments, but just started charging them accurately going forward.

Of course, that bill caused shock for some people, as they suddenly realised, "Oh s*it, I'm being charged accurately now, my bills are huge!!!"

In this customer's case, apparently they were also doing something bad with TLS tickets and not setting keep-alives - basically causing them to spam Firebase with connections:

https://news.ycombinator.com/item?id=14357270

Of course, there's the other issue of a communication breakdown which they said they're working on.