A picture of a angry cat is present in the background of every page. Many users reported on frwiki that they was surprised to see it.
How to reproduce this problem :
Available for logged in or not users, on every platform (I hope as it's in the css). Present on every small page where the white background behind the text does not hide the drawing.
In the patch I posted (https://gerrit.wikimedia.org/r/420384), I fixed the bug corresponding to this spot: the chat was no longer visible, it was simply deleted.
I would find it nice that rather than deleted it is put elsewhere (in the footer, in the preference page...) so that people are not surprised to see it there, among other possibilities.