Part of the Stack Exchange JS library includes functionality to create notification messages - the most common perhaps being "Thanks, we'll take a look at it" after casting a flag.
These notifications also come in "toast" style, which is a notification not fixed to a particular element on the page, but to the top-right corner of the screen.
On any other Stack Exchange site, these work fine. On Ask Ubuntu, the Ubuntu top bar comes over the top of them - I guess there's a z-index issue there.
And, to illustrate the overlap, this is the element as highlighted by Chrome's developer tools:
z-indexof101. One could argue that the toast's CSS should be fixed instead of the site's CSS; But I can't seem to think of any element in the page that may appear all the way up there, and that shouldn't show over that bar. I think the z-index property should be dropped altogether from the top bar's CSS.