Widest Path is a path, to its own subpages. This doesn't feel like good practice, but anyhow it works but for the following instance:
There was a subpage /2001 for which the path reference did not work.
Also, when trying to edit the page, it did not work.
Renaming the page to / Year 2001, enabled both the path reference and editing.
Maybe the page was just messed up and it is no structural bug of path-slash-number combinations.
Later: indeed it is not possible to create subpages with only numbers and spaces. How this page ever crept in, I don't know. So it has everything to do with the name of the page, not with the path. Shouldn't be reproducable.
On 8th January 2005 (I've just looked it up) I added the restriction that all pagenames and subpages must contain at least one letter. Since then editing/adding such pages is no longer possible. As the subpage's name '2001' suggests it was created/edited earlier.
I just did a search and found out that there are still 11 subpages that violate that rule. Will rename them now.