In Context UI, I created a content title of "Header / Footer" without really noticing it's a machine name.
This generates the edit URL of "/admin/structure/context/list/Header / Footer/edit"
Which leads to a "Page not found"
The simple fix if someone makes this mistake is to rename the "name" of the context in the database to not include / characters.
I suggest making machine names follow the other the typical list of I see in CCK - upper / lower letters, hyphen, underscore.
(A feature request to consider would be to add the CCK feature were it auto-generates the machine name as you type the normal name)