For people who use Apple's pro audio applications Logic and Mainstage, the applications were recently updated to versions that are fully 64-bit capable, which allows them to better take advantage of the hardware capabilities in Apple's computers. This enables users to run more plugins and other features at the same time without a performance hit. Despite this benefit, there may be some unforeseen incompatibilities when running in 64-bit mode that users should be aware of.
Like the limitation of the 64-bit OS X kernel not being able to load 32-bit extensions directly, the new version of Logic and Mainstage cannot directly load 32-bit audio units when running in 64-bit mode. To accommodate this limitation, Apple has included a "32-bit Audio Unit Bridge" feature which loads and translates plugins, but this may result in some odd functionality with some plugins. If this is the case, you should be able to still use the plugins by launching either or both of the programs in 32-bit mode. As with any application in OS X, you can do this by getting information on the application in the Finder and checking the "Open in 32-bit mode" option.
Users who depend on these applications may also have other concerns about their plugins and the application updates in general, which Apple has addressed in the following FAQ: http://support.apple.com/kb/HT3989