A service degradation sufficiently severe that tomorrow/Monday morning we'll have developers taken away from whatever they are working on to diagnose and fix the problem — whether that's a proper fix, an automated workaround, or a change in the monitoring system to reduce the severity of this problem from 'emergency'.

I took on a role with lots of outage alerts happening, at least a couple of times a week. I think my predecessor liked to play hero, by reporting every day or two what he'd 'fixed' (restarted) the previous evening/night. We had almost all of them fixed within a couple of months.