If you are using apply_filters('the_content', $ post->post_content);  to output your post content you might find that this function automatically ads and empty paragraph <p></p>  at the beginning of your content its starts with some sort of HTML tag different than a paragraph, something like an <ul>  o an <h1>  for example. This causes a nasty space to show up in your post, making elements missaling, etc.

To get rid of that empty opening paragraph simply add the following line of code before applying your filter remove_filter('the_content','wpautop');

So you end up with something simiar to this: