Interesting one this - I have no real idea what 'compact' means in any MTB-related terms - I thought it was a smaller BCD.
I took Jimmy's post as being what spacers between chainrings? I've seen all sorts of spacers fall out of cranks when whipping them apart - fatter washers, very thin washers - who knows?! I've generally only ever took apart to turn into single front ring though hence the not caring.