I'm using the Tonus 1.3 theme (newest version as of this post is 1.8) and came across a bug. The category links on the side bar work, except for the last one. The last category link is written wrong and leads to a 404 page. The following fix worked for me and may work for similar problems with other themes.
Go to Options, Permalinks and change the category base to read:
Just an FYI for future searchers.