It isn't - but it's the only `&` that's getting pulled into a title tag, which is what's causing the validation error. And, it looks like [we were setting `html_safe` on the title string](https://github.com/rails/rails/blob/master/guides/rails_guides/markdown.rb#L134), which was causing the title to be inserted without escaping.

`html_safe` seems unneccesary here, so I've removed it, which causes the `&` to be escaped. All guides validate correctly.