An anomaly has been found in Mac OS 7.6.1 that causes this behavior but does not affect all Macintosh models. Apple is working on a solution and it will be available in Mac OS 8. In the meantime, downgrading to Mac OS 7.6 will resolve the issue.
This freeze occurs immediately preceding the display of the shutdown message or the system powering off. At this point, it is safe to turn off the system.
This article was published in the 9 May 1997 "Information Alley".