Create an empty foo.tmpl file. Execute this code:
----------
package main

import (
        "html/template"
        "log"
        "os"
)

const (
        tmplFile = "foo.tmpl"
)

func main() {
        page := template.Must(template.New("page").ParseFiles(tmplFile))
        data := "bla"
        if err := page.ExecuteTemplate(os.Stdout, "page", data); err != nil {
                log.Print("Failed:", err)
        }
}
----------

The "page" there is a mistake, but the html/template package panics
(html/template internal error: template escaping out of sync) at template.go:68. If you
replace "page" with "foobarbaz" (or anything not naming a template
at all), you get a nil pointer panic at template.go:57.