Steps to reproduce:

1. Copy a URL.
2. Select some text in the visual editor. This text must include a line break.
3. Paste the URL.
4. Decide you don't like it. Type Command-Z to undo.
5. See the URL arrow icon still there.

First seen in Safari 9/Mac OS 10.10.5. It disappears after a little while (move the cursor, start typing, click somewhere...).