Some Unicode characters are not displayed properly, unless they are followed by another character of a similar script.
For example, Unicode character 25C6 (◆ black diamond) turns into a box unless it is followed by a Japanese character.
So text like "◆Text" instead displays the diamond as a box, but "◆あ" displays the diamond correctly.
Another example: "ↂℵ✁Text text" displays boxes instead of the three characters, but if you put a Japanese character at the end, ("ↂℵ✁Text textあ") it suddenly displays correctly.
I am on Windows XP, system locale is set to Japanese, and I'm using the Courier New font.
The problem seems to depend on what font is selected. If I select a font like Arial, Times New Roman, Comic Sans MS, Courier New, etc, I get boxes. But if I select a font like Microsoft Sans Serif, Meiryo UI, Lucida Sans Unicode, or DejaVu Sans, there are no problems.