When Apple released Security Update 2012-001 for Snow Leopard, it contained a nasty problem for anyone using older applications. The security update broke Rosetta, the software bridge that allows older PowerPC-based applications to run on Intel-based Macs.
It didn't take long for Apple to correct the issue. Security Update 2012-001.1 is available through the usual Apple Software Update service (select Software Update from the Apple menu).
Update: Apple has updated its website with Security Update 2012-001.1