The free convolution reverb LAConvolver AU (
http://audio.lernvall.com/) seems to cause lockups (GUI freezes). Not a serious issue, but it really sounds great and one can load a miriad of IRs from the net. A definite secret tip! Good convolution reverbs are expensive.
Here's how to reproduce: Hammer on space bar while frequently selecting different start positions on the time ruler. One audio track is enough.