Double-Encoded URLs
It is possible to get double-encoding in the URLs for some page titles, which results in an inaccessible page. It seems this only occurs for special characters which fall in the ASCII range and thus have an HTML entity equivalent, like a-umlaut.
I have already fixed the offending DM, but before we close this, need to add an integrity tool to rebuild URLs, and need to have the upgrade rebuild URLs to correct any previously damaged items.