System admins can customise space-specific velocity templates to make, eg, the view-page layout different.
Belatedly I've realised this is a very bad idea, as when you upgrade, the modified template will be retained, even though the default template has most likely changed. What you'd need to do is diff your changes with the original template, then reapply those to the new template.
As no warnings are giving, most likely you will forget, which will lead to subtly broken pages, or broken plugins. I just spent a day working out why the sticky notes plugin wasn't working.
So what's the question... I guess, should this even be possible? I would say not, it's never going to end well.
Also I think the upgrade should draw attention to customised templates and get you to check them all.
