Mostly things that smelled like memory leaks (pull a matte, leave it for a while, come back and tweak it => long coffee break). Also, restoring from idle seemed to be the hardest on Snow Leopard.
Which keyer? Primatte's had a few issues in 6.3 and previous.
on 10.6 (and 10.7), memory allocation/paging is pretty atrocious if you haven't got much free mem left.
Basically, OS X prioritises paging to disk over freeing up Inactive memory (which isn't actually being used for anything at present), which is pretty crap. So it's very easy to make it page when it shouldn't when you allocate loads of memory, and the system will generally just grind to a halt.
Apple fixed this in 10.8, so now it will free up Inactive memory first, before it starts paging.
But neither of these explain any crashes within Nuke.
