When emacs is built with cairo I see small display artifacts from time
to time, in particular there are small thin vertical bars, for example,
with the default theme (starting from emacs -Q) I get those bars in the
mode-line and they are always in a cyan color, but it is possible that
other occurences can be observed even with emacs -Q.

In real world usage I have seen these bars in regular buffers, too,
usually they are red and/or blue but that might depend on my theme.

A recipe to produce them (on my machine) is:

1) emacs -Q
2) M-<
3) C-space
4) M->
5) M-w
6) C-y (repeat ten or twenty times)

Then move around in the resulting buffer and after a bit of cursor
movement the vertical lines appear. They are not permanent and often
disappear when the buffer is refreshed.

This issue is not present when emacs is built without cairo.

I attached a screenshot of emacs showing a small vertical cyan bar in
the mode line. produced with the above recipe.