Most browsers will return an error page when you click on a multiple word tag (ex: book fun, mod post, back to school). In order to keep that from happening, you will need to change those tags.
Now, before you go off screaming about having to go through all your entries to find the multiple word tags, LiveJournal has made it easy to edit them. Click here to manage your tags.
Since most browsers thing a space between words means AND (example: books+fun, back+to+school, etc), you will need to put an underscore (_) between all the words you want to keep seperate (ex: book_fun, back_to_school).
I know tags are a great way to keep your entries organized, so just remember to use the _ when you're using multiple words.