I've found that, when encoding a struct into JSON, if there is a space after the colon character in the tag for the field in the struct in question, then the field will not be renamed. Below is some very simple code demonstrating what I mean.

package main

import (
    "encoding/json"
    "fmt"
)

type Person struct {
    Name string `json: "name"`
    Age  int    `json: "age"`
}

func main() {
    p := Person{"Alice", 29}
    bytes, _ := json.Marshal(p)
    fmt.Println(string(bytes))
}

As you can see, in the above code I've made the apparent "mistake" of adding a space in my JSON tags.
If I run this code, the output I get the following output.

{"Name":"Alice","Age":29}

Now if I change the struct definition to the following

type Person struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
}

and then recompile and run the code, I get the expected output

{"name":"Alice","age":29}

What's interesting is that this isn't a problem when decoding JSON. Reintroducing spaces after the colon character, and switching to decoding from bytes, you'll see that the struct is populated properly even though I have switched the order of the fields in the JSON. I take this to mean that the decoder understands the JSON tags even with the space when decoding.

package main

import (
    "encoding/json"
    "fmt"
)

type Person struct {
    Name string `json: "name"`
    Age  int    `json: "age"`
}

func main() {
    data := []byte(`{"age": 29, "name": "Alice"}`)
    p := Person{}
    json.Unmarshal(data, &p)
    fmt.Println(p)
}

The output I get is, as expected,

{Alice 29}

I should note that I've also experimented with using a Decoder and Encoder instead of Unmarshal and Marshal. They suffer from the same problem (and I assume share a similar implementation to their respective counterparts).

I tested this running Go 1.5.3 on the latest OS X. The output of go version is

go version go1.5.3 darwin/amd64

It seems to me that the encoding/json's decoding and encoding functionality should behave the same with regards to how struct tags are treated. I should be able to have spaces after ":" in the tag and still have struct fields encoded such that the right keys are used, especially since decoding isn't affected by this.