This is how I noticed this:

1. Go to a forum and click to add a new discussion.
2. Click in the "Message" editor.
3. Type some text, press "new line" (Enter) and write some more text. The first line has a different format than the second one.

The reason is that when I start typing in the editor, the text is not inside a <p>. However, adding a new line adds a <p> to that line.