This isn't a bug in Go, but it does cause breakage elsewhere. I'm not sure what the fix should be (if any).

What version of Go are you using (go version)?

1.7.5, 1.8

What operating system and processor architecture are you using (go env)?

Linux amd64

What did you do?

Ran this program:

package main

import (
    "encoding/json"
    "fmt"
    "log"
)

func main() {
    b, err := json.Marshal(5e20)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(string(b))
}

In 1.7.5, the output is 5e+20.

In 1.8, the output is 500000000000000000000.

Both are legal JSON, but the latter breaks BigQuery:

InsertAll googleapi: Error 400: Invalid JSON payload received. Unable to parse number.
[{"json":{"MyFloat":50000000000000000000
                    ^, badRequest