On page 3 (and several other pages) there are some seemingly broken/empty rectangles (tested with Evince and Okular): http://www.idi.ntnu.no/emner/tdt4171/slides/Lecture12.pdf
Xpdf handles it correctly, fwiw.
If it happens in both Okular and evince most probably it is not Splash specific
It works for me with both splash and cairo.
Both cairo and splash render it correctly, okular is misinterpreting the annotations though, reopen the bug against okular please.
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