Sorry for the delay. I was at this site, http://hedonblog.wordpress.com/ , which is the one linked to your username. Please always provide a link for use when you have multiple sites.
I see nothing really that should cause this, but the thing is, the structure is set up to align the posts in the right and left column, and the post at top left is longer than the one at top right.
Using Firebug I deleted the last paragraph in the Scary post and the misalignment went away, so this confirms the theme is shifting that second post on the left down so that it can align with a post in the right column.
I'm not sure there is anything that can be done about this and still keep posts aligned properly side by side, but I would contact staff on it and see what they say.
http://en.support.wordpress.com/contact/ and do a search for anything, then scroll down to the bottom, click the "no" button and the contact form will appear.