When I visit a nonexistent subpage, for example
it reports that the page does not exist and that "Titles on Wikipedia are case sensitive except for the first character."
However, the page
does exist, and the only difference is the capital A in Archive.
The expected behavior is that the first character after a slash in a subpage name should not be case sensitive.
Before changing this, have a look at how many pages exist where there ARE two different pages differing only by the case of the first letter after a slash (or multiple case-changes-after-slashes, for nested subpages). Usually one will be a redirect to the other one. Please be sure to keep access to the one that is not a redirect.
If you think my intuitive expectation about the behavior is not at all grounded in reasonableness, we should at least update the "page not found" documentation to note that ALL characters, even the first, in subpage names are case sensitive. I'd rather just have the intuitive behavior, though.