package main

import (
    "log"
    "net/http"
    "net/http/httputil"
    "net/url"
)

func main() {
    u, _ := url.Parse("http://localhost:8002/";)
    go http.ListenAndServe(":8002", http.HandlerFunc(echo))
    log.Fatal(http.ListenAndServe(":8001", httputil.NewSingleHostReverseProxy(u)))
}

func echo(w http.ResponseWriter, req *http.Request) {
    w.Header().Set("Content-Type", "text/plain")
    req.Write(w)
}

There is an echo server running on http://localhost:8002/
that prints the request back, and a proxy on http://localhost:8001/
that proxies to http://localhost:8002/.

I expect that loading either port should be the same, but they're not.

coma=; curl http://localhost:8001/
GET / HTTP/1.1
Host: localhost:8001
User-Agent: curl/7.21.3 (x86_64-pc-linux-gnu) libcurl/7.21.3 OpenSSL/0.9.8o zlib/1.2.3.4
libidn/1.18
Accept: */*
Accept-Encoding: gzip
X-Forwarded-For: ::1

coma=; curl http://localhost:8002/
GET / HTTP/1.1
Host: localhost:8002
User-Agent: curl/7.21.3 (x86_64-pc-linux-gnu) libcurl/7.21.3 OpenSSL/0.9.8o zlib/1.2.3.4
libidn/1.18
Accept: */*

coma=; 

Remember that the 8002 server is echoing back the request
in both cases.  In the proxying (first) case, the proxied
request still says Host: localhost:8001.  That seems wrong.