Several OS X 10.6 Snow Leopard users are reporting that Power PC-coded apps no longer work after applying Security Update 2012-001.
The Apple support boards are full of stories about Rosetta apps crashing when any command is selected from the menu bar. Complaints seem heaviest from those running older versions of Microsoft Office, Quicken, FileMaker and Photoshop. Some users have recommended unofficial workarounds for the problem (attempt at your own risk), and it isn't known if Apple will fix the issue.
Rosetta was deep-sixed in Lion, but was working fine in Snow Leopard until the security update was issued on Wednesday. Rosetta was introduced in Mac OS X 10.4.4; the code-translation layer allows apps that require a PowerPC Mac to operate on Intel-based Macs.
Meanwhile, this week's Lion update had its own share of issues, with a number of users seeing strange graphics on screen and getting caught in a loop where applications would not close or re-open. Fortunately, it seems that applying the 10.7.3 combo update fixed the issue for many users.