Use https://en.wikipedia.org/w/index.php?title=Wikipedia&veaction=edit as an example, and open the category editor. The sortkey for the category "Wikipedia" is just a space (to put the article at the very beginning of the category), but you can't see this, unless you put the cursor into the text field for the sortkey and move it to confirm that there really is a space. A similar issue exists for sortkeys that start with a space (used in some cases when there are two or more main articles in a category), where you can't distinguish the leading space from padding.

In these cases instead of the raw sortkey a label could be displayed, telling the user about the otherwise invisible space.