I frequently use comments in the "source code" of my nodes, but I have noticed that sometimes it causes the resulting preview page to display in an odd way, with the chatterbox right below the post, full screen, and no submit button. Perhaps the "valid HTML" parser is choking on the comments? To duplicate this, preview a node and add a comment as the last line, like this:
<-- I am a comment. Hear me roar. -->