I didn't report this earlier as it is such a subtle bug and difficult to reproduce. I first noticed this many months ago, likely right after the comment update that split the "add comment / show all comments" button in two.
I often browse "hot network questions". I don't have an account on all SE sites. If I leave a tab open for a while and somebody adds a comment to an answer or the question, the "show more comments" link on that particular answer does not expand the comments (instead it acts sort of as a link to #top). A page reload fixes this however, and this only occurs for unregistered users.
I don't have any kind of visualization of this because I can't reproduce it on demand for obvious reasons. (This bug always reproduces when somebody adds a comment, but getting somebody to add a comment on demand is more challenging.)
This report is a duplicate of this question. My attempt to clarify the title to better reflect the content was rejected (I understand why, as the question hard to comprehend with all the edits, and I refrained from changing the content too much), and I figured posting a new question is the best way to clarify the issue, and bring to attention the fact that this regression is bordering on annoying yet remains unfixed many months after introduction.
I use Opera, but it's likely not browser specific as others have reported the same issue on Firefox and Chrome in the other question.