Now that the juce_audio_plugin module embeds Apple's CoreAudio support files (awesome change btw! one less thing to set up!), it now also embeds the corresponding bugs in those files:
When building plugins with current JUCE in Xcode using OS X SDK >= 10.9, those plugins will crash and fail validation when used in OS X 10.6.
More details in this old thread: http://www.juce.com/forum/topic/aus-build-sdk-109-crash-older-auval-work-around-request