If you choose to paginate your comments then you probably have this set to occur after 20 comments, as this is the default value in WordPress. If WordPress comments are paginated and set at 20 comments a page with newer comments displayed at the top, a small problem occurs when there is slightly more than 20 comments.
For example, if you have 21 comments WordPress would display 1 comment on the first page and 20 comments on the second page. This plugin addresses this problem and reverses it to display 20 comments on the first page and 1 comment on the second.
If you paginate your comments and have noticed this problem then I recommend trying this plugin out :)