Closely related to tests are monitoring the production environment. Every time there was an incident and it wasn't visible at the source (whether it was found d

Closely related to tests are monitoring the production environment. Every time there was an incident and it wasn't visible at the source (whether it was found due to customers complaining or conversion rates dropping), no matter how unlikely it seems for it to happen again (we made a unit test so we should be safe now, right?), always monitor for it. This might be my ops side speaking, but often undesired behavior aren't outright bugs (a value not being set is expected, but that empty value led to an empty template which led to the frontend framework not reading an event might not be), the same class of issues can be monitored for. This is a combination of logging and monitoring, of both events (where the application checks for unexpected results) and of state (where something external to the application reacts to for example things like active users without login sessions).