Part 1:

1. Start creating a content such as an Article, for instance (as long as it has a richtext fieldtype);

2. Copy some URL address onto clipboard ( https://www.google.pt ), and try to paste it onto a richtext field.
It won't work.

Part 2:

3. Now copy something not an url ( this is so not a url), and try to paste it also onto the richtext field:
It will paste the copied text alongside the previously copied URL, before the text.

This was tested on Firefox, Opera and Chrome, and was tested on 1.5 and master.

On 1.4, the issue did not manifest.