I have had real problems trying to go from website.co.uk/category-slug-2, where WP was adding a '-2' for no apparent reason.
> Check all other categories, tags, pages, posts, and custom MENU'S
There is a lot of info in the forums about this issue, none of which helped me - so I wanted to share my solution.
The reason it happens in because the term you are using is already in use by another element elsewhere in the site. For me, I was trying to create the category 'best-mortgage-rates', and the -2 kept on adding despite me clearing all articles from trash like other posts suggest, because it was taken up by a custom menu name.
I changed my custom menu name to something else and now it's free it works fine.