Running on OSX as well and did not experience any problems. Could you clean the demo (ant clean.demo) and re-install it once again (ant install.demo.noeclipse). Then start the demo environment (ant start.demo.noeclipse) and then perform the activities that lead to a failure. Once you encounter the error please make a copy of server.log and upload it here. Additionally if you could describe step by step what you did to encounter this error. That can help to try to reproduce it on my side so hopefully that can give us some useful information.
Cheers