This was an interesting symbol to choose for an internet explanation. It took me a while to realise that the rectangle is actually the symbol that I'm supposed to see, rather than a missing glyph.
I see, it is indeed showing a missing character symbol on your computer. The actual character is much more square than that, in addition to the strokes I mentioned. (http://i.imgur.com/s6bW7u0.png)
Doesn't help that at least for me it gets rendered differently in the title on HN and the actual article.
That said, I expected something Japanese related, given that it's Candy Japan, but you'd think common fonts would have different looking characters for “no glyph” and the kanji for mouth.
Another problem is that if the character appears in isolation, it's hard to tell whether you are looking at 口 (mouth) or 囗 (the "box radical" kanji, or "kunigamae").
Ha, for me, they look quite different in the font used for editing, but not so much in the rendered comment other than a small difference in size.
