There's a bug in the OpenGL system in Mac OSX that causes preview tearing. The next version of After Effects, After Effects CC (12.0), works around this bug.
Running after effects cs6 126.96.36.199
with the most recent update
operating system OS X 10.8.3
3.4 ghz i7
8 gb 1600 mhz ddr3
nvidia geforce gtx 680mx 2048 mb
1 tb fusion drive
quicktime is installed
was not running any other software
no 3rd party effects or codecs
doesn't seem to happen when output to quicktime
The bug that I referred to doesn't affect output. Just preview on Mac OS.
Adobe After Effects CS6 (11.0.3) has been released. This update fixes many crashes and other bugs, including this OpenGL tearing issue.
Also, this fix is in After Effects CC (12.0), as mentioned above.