I have a site using the Minimum Theme from StudioPress. I noticed when a post has comments disabled, that the bar starts to float about halfway through the featured image (which is before the .entry-content) instead of waiting until the bar is reached and then floating it. This means the bar jumps up to the top when you start scrolling instead of staying put until it is reached.
The issue goes away when comments are enabled on the post.
Here's an example:
The bar at its normal position within the post: http://imgur.com/MzTERSH
Scrolling down a little further, but not yet reaching the bar, it jumps up to the top: http://imgur.com/JGPAt9J