If you double-click a UI file to open it, then hit the escape key, it switches to the XML editor view, which is not very useful (well maybe if somebody likes to edit XML a lot by hand, but I do it very rarely myself). For me this is a reflex because I usually hit escape twice to get back to my C++ code: close the debugger, the error output from the compiler, and so on. In Designer it's disorienting that it doesn't work. Then I usually correct this mistake by hitting control-W to close that file completely; because next time I want to open the UI file, I will just double-click it in the project tree again. So we could say that the XML editor is too sticky: the workflow naturally leads to me seeing the raw XML, which I don't normally want to see, and it takes an extra step to get back to what I want to see.
If you see that XML showing up in my presentation when I'm trying to show Designer, that will be why. (I noticed this happening a lot while rehearsing for QtWS, and people complained: why did I show the useless XML code so much, so I had to explain why.)