Json:

{ Some_ID: 4 }

Go:

type Example struct {
    SomeID int "Some_ID"
}
...
data := <above json as []byte>
var e Example
json.Unmarshal(data, &e)

Given the above json and struct, the value e.SomeID will not be filled with the value 4.

From the first of three rules listed at http://blog.golang.org/json-and-go:

[Unmarshal will look through the destination struct's fields to find] An exported field with a tag of [the json field's name]

If I change the struct definition to

type Example struct {
    Some_ID int "Some_ID"
}

Then, after Unmarshalling, e.Some_ID will contain 4 as expected.

It appears that when an underscore is involved in the field in the json, tagging the struct's field with an underscore when the struct's field does not contain an underscore will not marshal that field. If the json contains an underscore, so must the struct's field and the tag is useless.