A] Using the extends override for ui items can result in blank tabs on forms or spaces in the action bar.
I have provided an export of a dialog which illustrates the issue. This dialog takes stkTextImage and overrides the image tab to make it essential an stkText component.
Here are the steps to reproduce:
1. Import the attached dialog into: confg:/modules/standard-templating-kit/dialogs/components/content
2. Configure the stkTextImage component to use this dialog
3. Try adding this component to a page
4. Notice that the form in the dialog still has 2 tabs. The overridden image tab is blank but should be completely removed.
B] Another place that strange behavior has been observed is with the action bar. For example, if you wanted to create a subApp that has a similar actionbar as another subApp but removes a couple items. If you add the extends override at the individual item level it leaves a space in the actionbar rather than suppressing it completely. We do something similar in this exercise: http://wiki.magnolia-cms.com/display/TRAINING5/Exercise%3A+Create+JCR-Browser+Sub-App (Part: 2 #2)