Example link: https://de.wikipedia.org/w/index.php?title=Jackie_Mittoo&diff=161316419&oldid=132611209&diffmode=visual
If you have diff + preview enabled, and when there is no content in the diff part of the visual diff, but there is content in the annotation column, then the section has no height, and causes the annotations to overlap. A clearing of the floating is required (can't we just use flexbox here btw, with floating fallback for old crappy browsers ?).
This might also occur for visual diffs of VE changes, I haven't tested that.