Adding a New Style (where no style exists) Note:- Do not attempt to apply a style where a style has already been applied - see section below on removing a style
More than likely a style already exists and you have attempted to apply a style over an existing style. What happens with styles behind the scenes is that the styles are nested. The more you attempt to apply the style the greater the problem becomes. You can see this by using the html editor instead of design.
If you get into this situation either delete the heading and start again or keep selecting the heading and apply the steps to remove the style until the style is removed.