I would be interested to read bug reports from people who thought they had a complete enumeration but learned in production that their enumeration was incomplete. I've never seen it myself.
An easy way to get that is when you use a third-party library, update that to get some new feature, and get a new enum value for free, say because the html parser library started supporting a new node type or added a new error type (the latter will be rare in go because of its lack of sum types).
Ideally, of course, the release notes of the library would spell out the issue, and you’d read them, but even then, making sure you fix this everywhere is way easier if the compiler refuses to compile your code if it doesn’t handle the new case.
This is relatively common with naive int / string to enum type conversions, especially if the int or string are given as input from an RPC mechanism of some sort.
This type of pattern can be used to force other developers to check if the value is garbage before calling $BUSINESS_LOGIC that uses the enum
At my current day job we have many, many such "enums" that slowly expand over time. I've seen many bugs from new values not being taken into account in the several places one has to remember to double check. Sometimes testing catches these, sometimes it doesn't.
