I don't think I've ever worked on a project of this magnitude before so this was a bit of a revelation to me: when your customer base reaches a certain size, you stop having edge cases.
Mistakes, bugs, incompatibilities, and related issues that used to affect a handful now affect hundreds. 1% is real number now. This requires some organizational change. More caution, more testing, more contingency planning, more disaster planning.
This is likely the reason why customer service for large services is so difficult (to the point that almost no one gets it right): every little problem is actually a big problem. See also why writing software is easy but writing stable, scalable, engaging, polished software is not something for a weekend hack and Too Big to Succeed.