Deleted comments are handled as comments with no height. You can still see lines separating the comments.
The comments initially appear like this: http://puu.sh/9vneK/8e1ab8bbad.jpg
When expanded: http://puu.sh/9vnfn/9a6a2758e3.jpg

Not sure that's a bug.

It indicates when and where a comment has been deleted in a thread.

This can help with comprehension of the remaining comments.

If that was the case, there would likely be some sort of indication. I assume it is a glitch because there is no way to know what it signifies without already knowing about the glitch. However, it would be a good idea to display a message indicating deleted comments.