In Logic 9.1.8, when
[list]
[]Using a MIDI controlled effect (such as the Juce demo plugin)[/]
[]On a midi track with the “Record Button” (“R”) enabled (the track responds to the user playing on a MIDI keyboard)[/]
[]With the playback paused[/][/list]
Even though playback is paused, Logic still calls the effect to process samples, but it doesn’t give it new input buffers. So effects get as input to their processBlock whatever they outputted previously, often resulting in unpleasant sounds.
A suggested fix is to zero the buffers in this case adding a
for (int i = numInChans; i < numIn; ++i)
FloatVectorOperations::clear (channels [i], numSamples);
at juce_AU_Wrapper.mm:764
UPDATE: Actually I hadn’t tested my solution well enough and it doesn’t really work 
So consider this just a bug report with no solution currently
Cheers! Yair