I've noticed that the max-width:50em media query has the postarea margin just a bit too small. It's most obvious if you have an image in the post. It will pop down below the left column poster area leaving a gap at the top of the post. Increasing the postarea left margin from 8.0em to 8.2em fixes this problem.
ETA: Actually the 8.2 is font size dependent. I'm not sure how the margin was chosen since it's considerably smaller than the min-width of the posterarea.
Sorry, I'm not entirely sure here what the problem is... :'(
Could you please post a picture? (I'm sorry, from time to time I really have problems follow theme-related discussions... :-[)
Testing just now this seems to have been fixed with the changes to poster.
Great! :D
/me likes when things fix by themselves. lol