https://en.wikipedia.org/wiki/Template:Sfn is supposed to be placed right next to the sentence you want to reference.
For some reason though when adding it unwanted lines are added as well, so that it looks like this, rather than like this.
This seems to happen at en.wp and at ja.wp as well (where the issue was reported). I'm not familiar enough with the template to find out why sometimes I manage to add it correctly, and sometimes I don't (same browser, OS, page etc.).
I can say that even when it's added correctly, it will look "wrong" on the page, but it will show up fine both while reviewing changes and saving.