UX issues aside, some leaks will sink you more quickly than others. These are stop-everything moments that you want to know about immediately, instead of next time you check in on things.
Some people call this emergency handling. We call it setting up your safety nets.
Establish a process of alerts for your payment processor API uptime, and monitor your dunning system for critical failures.
Did a recent code push take down your card update pages? Can you detect who tried to access those pages during the downtime?
When your email service provider has downtime, or their queues back up…are emails still getting sent out?
If these kind of hiccups routinely happen without emergency handling, you will routinely miss out on opportunities to renew subscriptions.