the wordpress.com demo version of your theme seems to have a problem with the infinite scroll feature. I don’t see any mention of it in the forum. Are you aware of it?
At first, with the home url, everything is ok (minus a slow loading time at the beginning).
When reaching the end of the first page, the url change (the suffix “page/2/” is added to the home url) and the page 2 posts are all in the left column and the first ones are superposed with the previous posts.
Same thing for the next pages (all in the left column).
I see that problem on desktop (OS X 10.9.2, Chrome and Firefox, last versions), iPad and iPhone (on Chrome).
Thanks for reaching out with the notice. We are working to fix an issue right now that should fix the issue. WordPress.com just updated to the latest beta release of WordPress 3.9 and there was a conflict. So it should be fixed soon.