If you are seeing something strange where the Simplified Chinese character 门 (men2, U+95E8) should appear, there is nothing wrong with your system or app. It turns out that the Hiragino Japanese fonts supplied with OS X have an unusual version of this (the left character in the graphic below), which I've been told can be found in Japanese handwriting (see example 2 at this page ). Normally Japanese computer text will have the Traditional character 門 (U+9580) instead.To fix this display issue, so you get the character on the right side of the graphic, make sure that in System Preferences/Language & Text/Language you have Japanese lower on the list than Chinese, or use the Edit button to uncheck the box for Japanese.
One might consider this to be a bug in the font. Following the Unicode Standard, this version of the character should be invoked by the data sequence U+95E8 U+E0100 (or E0101), where the second element is an ideographic Variation Selector.