I have a habit of opening the same link multiple times, since it's sometimes easier to click the link in the thread than to Ctrl+PageDown to switch tabs.
In this case, I had it open, came back to the HN thread to leave a comment, opened the site again to look some more (twice?), came back to the thread and read someone else's comment about consistency, went back to look at what they were saying, and that's when I noticed the icon was different compared to the other three tabs of it I already had open.
Yeah, it's pretty bizarre. I'm currently able to reproduce both icons being served (in Firefox on Windows) by doing a hard refresh (control+shift+R) twice in a row, then doing a normal refresh (control+R) twice in a row, repeating the sequence a few times. I'll consistently see different icons throughout this process.
In this case, I had it open, came back to the HN thread to leave a comment, opened the site again to look some more (twice?), came back to the thread and read someone else's comment about consistency, went back to look at what they were saying, and that's when I noticed the icon was different compared to the other three tabs of it I already had open.