The other day I dragged out my old Mac Color Classic (LC 550 
motherboard, 36mb RAM, System 7.5.5) and tried to install NetBSD 1.6.2. 
  A couple of years ago I had it running just fine with 1.6.  Now, I 
find that with pretty much the same setup as before, I get a kernel 
panic on first boot caused by an MMU error.  When I start MacOS with 
extensions off, or just the Memory control panel turned off, the 
problem goes away.  Anyone else run into this?

Tom