Hello Matthew Snyder,
Please take a look at Sitefinity's UserManual, section Extensionless URLs.
Possible reasons for this error are:
-Wrong setting of IIS for extensionless urls.
-there is a something else on the website that misleads the url parser.
sf404 should be configured in IIS as Error page/WildcardMapping or in the web.config as custom error. This is the correct setup.
I am not able to reproduce the issue locally.
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