In the recent Tidbits issue Matt Neuburg explains how Apple twiddled its “New Document Model” in Mountain Lion as opposed to Snow Lion. In brief: DO TICK the two check boxes I encircled in orange in the screen shot below (System Preferences>General). Otherwise you may get burned (I have) by “experimenting” with the contents of an old file which should actually stay unchanged. With these settings the advantages of “New Document Model” may begin to outweigh its disadvantages.