I got C3D 2008 to crash. I used LAYISO to isolate some linework, and then tried to create a parcel from it. Things got VERY strange.
Haven't had enough time to do rigorous testing on it, but my initial suspect is that it has something to do with the change to LAYISO. Now, instead of turning the other layers OFF, it locks the other layers. This causes them to be displayed dimly, with half-highlighting. I don't think I really like this, because a primary reason for using LAYISO is to make sure my OSNAP is going to the right object, and it's still possible to OSNAP to objects on locked layers.
Anyway, my initial suspect is that something about having most of my layers locked caused an issue. I've had other C3D commands complain when I try to run them after doing a LAYISO, because they try to access data on a locked layer (not sure which one). But when I tried to create parcels, I got very stange behavior. It was supposed to be four parcels, and I got labels for four parcels, but they all appeared outside of the site and superimposed on top of each other, and three of them had the same areas. And only two parcels showed up in Toolspace. I finally decided C3D was in a bad state and tried to shut it down, but it crashed instead.
I'm not sure this problem had anything to do with locked layers, but that's my initial guess.