The plugin uses nonces for form security. You can read more about them here.. http://codex.wordpress.org/Glossary#Nonce
This typically happens after you install and are testing. The browsers cache the pages. You go back to the page, wordpress serves up a new nonce but the browser serves up the cached page with the old nonce and you get a security check. I have only run into this problem when I'm testing, going back and forth, back and forth etc.
This will never happen to a typical visitor.
If you clear your browsers cache, that should solve it.
Also, if you are using a cache plugin like W3 Total Cache or WP Super Cache it can happen in reverse.