http://xquartz.macosforge.org/trac/wiki

macosx에 포함되어 있는 X11 (Xquartz)가 불안정한 관계로 가능하면 업데이트를 해 주는 것이 좋다.
자세한 것은 위 url을 참고

Intro¶

The Xquartz project is an open-source effort to develop a version of the X.org X Window System that runs on Mac OS X. Together with supporting libraries and applications, it forms the X11.app that Apple has shipped with OS X since version 10.5.