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.