For example (it happens on other pages too):
The button for "Add User" (link to /admin/people/create) disappears after a few seconds of loading the page.
I have 2 demo installations (one local and one on the internet) and it happens on both sites.
Looking at the html of the page, the code is still there for the button to be displayed but for some reason it disappears.
As I say, it also happens on other pages too.