Why is my new viewlet showing up in other skins on other plone instances?
When creating a custom skin/theme a viewlet (registered in browser/configure.xml) is showing up in instances, that don't use the custom skin.
You probably don't have the layer=".interfaces.IThemeSpecific" line. Ensure that your entry in browser/configure.xml looks like this:
<browser:viewlet name="myskin.myspecialviewlet" manager="plone.app.layout.viewlets.interfaces.IPortalHeader" class=".viewlets.MySpecialViewlet" permission="zope2.View" layer=".interfaces.IThemeSpecific" />
You may need to restart Plone after the change is made.