On chrome:
1. Create a new page
2. Insert a horizontal rule as the first thing on the page
3. Press enter
Another HR is created every time you press enter. You can delete all of them except the first one which cannot be deleted.

On firefox:
1. Create a new page
2. Insert a horizontal rule as the first thing on the page
3. Press delete to try to delete the HR. You can't
4. Now press enter. Every time you press enter a new HR is created.

Other browsers have similar issues.