With packages like this one:
If the trailing slash is not in the URL, the site returns a 404 error because of the .cs extension on the request. The Packages page (and others) produce URLs that don't have the trailing slash. The gallery should always produce package URLs with trailing slashes to prevent 404s from links we ourselves generate.
This is either a dupe of #657 or a logical follow-up work item so I think I should take it.
Appears to indeed be a straight dupe, so closing.