[HTTP on HTTPS]By mixing http with https you'll endup with mixed content in your site(in websites) hence the green code in your site that indicates your site is secure will be replaced with a site-is-insecure for that specific page that contains a http call.
Its insecure because generally you'll be serving content on a secure protocol via an insecure tunnel(info on that page can be accessed via a HTTP link) . Anyone with a little know how of how things work on the web can easily intercept the data and even maybe rewrite it for his/her/its own benefits. Imagine if for example Google was serving its services via HTTP, think of how many phishing grounds that would create(hackers trying to get sensitive data), users would no longer be able to know which is the actual secure site and they would end up loosing if not all at least a third of its customers.