I think I found generic pattern of this issue. It seems to happen when some single tag that is not closed (like hr, img etc) has no “air” around them. Adding two line breaks before and after those tags solves the problem. Interesting thing is that there may be more than one problematic tag on page. They look the same. But you “fix” only one of these and the issue is gone.
Not sure if it helps you anyhow but you can still play with the blog post I referred above. Just minify the markup to remove spaces and line breaks between the tags and see what happens.
