You should rarely reach for panic. Panics are not like exceptions, really. It's very frowned upon for panics to pass API boundaries. Use errors always, unless the state of the world is irrecoverably broken.

It's difficult to come up with examples of when this may be the case. Often it's a "you're holding it wrong" kind of thing. For example, a common idiom is wrapping something like

    func Parse(s string) (Foo, error)

to be usable in contexts where error handling isn't possible, such as variable initialization.

    func Must(f Foo, err error) Foo {
        if err != nil {
            panic(err)
        }
        return f
    }

which could be used at the global level like so:

    var someFoo = Must(Parse("hey"))

This is acceptable because this code is static and will either panic at boot always or never.