Hello!
Some .sit files are compressed on a computer that are about to suffer from directory damage resulting in a incorrect resource fork.To open these files do the following.
Double click Stuffit Expander icon
Now from whitin Stuffit select File menu and then the file that you would like to expand.
Do not use Drop Stuff that one is for compressing files and not to expand them.
Also you should ofcourse visit Stuffit's site and download the latest version of the Stuffit Lite package.
Stuffit's site