If you've ever gotten this annoying error when trying to restart or shut down System 6 because StuffIt Expander 4.0.1/4.0.2 was running in the background, you can fix it by opening StuffIt in ResEdit and changing the existing 'mstr' resource 100 to have an id of 103.
'mstr' resources tell MultiFinder which menu items to generate fake clicks for in order to quit or open another file from Finder when the application is already open in the background. StuffIt needs one because its File->Open menu item is named File->Expand... but Aladdin goofed and numbered it 100, which is supposed to be the name of the 'File' menu if it's not named 'File'. By changing it to 103, it lets MultiFinder know how to remotely open a file and also unbreaks its ability to select File->Quit at shutdown time.