This isn't a bug in zope.app.form. The SimpleVocabulary was behaving as designed. Launchpad just needed a simple subclass that would ignore bad values and use a default instead, which has now been done.
This isn't a bug in zope.app.form. The SimpleVocabulary was behaving as designed. Launchpad just needed a simple subclass that would ignore bad values and use a default instead, which has now been done.