I’m working on a few Firefox extensions, and I came across a pretty slick tool that generates a skeleton for your extension:

Firefox / Thunderbird Extension Generator

You put in some basic info about your extension and it generates a zip file with the skeleton for your extension. For some reason, whenever I double-clicked this zip in OS X and Stuff-it tried to expand the zip, I got this error:

An error occurred attempting to expand ‘extension.zip’. Format error. Error #17540.

After about 15 minutes of frustrating attempts to rebuild the extension all resulting in the same error, I just tried opening the file from the command line:

$ cd downloads
$ unzip extension.zip

This did the trick.