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

$ go version
go version go1.14.2 darwin/amd64

What did you do?

package main

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

func main() {
	p := "404 not found"
	var m interface{}
	dec := json.NewDecoder(bytes.NewBuffer([]byte(p)))
	dec.UseNumber()
	err := dec.Decode(&m)
        // the err is nil
	if err != nil {
		fmt.Println(err)
	}
        // 404
	fmt.Println(m)

	err = json.Unmarshal([]byte(p), &m)
        // the err is not nil
	if err != nil {
		fmt.Println(err)
	}
        // 404
	fmt.Println(m)
}

What did you expect to see?

both m is 404, but the first err is nil, and the second err is "invalid character 'n' after top-level value". Maybe both should reutrn error?