Templates for directives with replace mode to true can only have one root element, otherwise this error occurs. However, comment tags are also counted in that check. So for example, the following:

app.directive("superman", function() {
  return {
    restrict: "E",
    template: "<!-- Superman directive --><div>Here I am to save the day</div>",
    replace: true
  }
})

http://codepen.io/anon/pen/bazyI

Causes an error. Maybe this will be considered a won't-fix but our designer guide couldn't comprehend why the template suddenly stopped working when he added a comment. Perhaps comment tags should be ignored from that count.