StuffIt Expander 5.5 won't open .sit archives that don't have the right file/creator types, which often happens if the file was transferred through a non-Mac system. There's no option to "show all files" like there is with the full version of StuffIt. That's stumped me more than a few times.

Very small claim to fame: I went to college with Raymond Lau, the creator of StuffIt.