Skype appears to be unable to handle this and tries /favicon.ico%3F0
which triggers a 404 errors. The user agent is the following:
Mozilla/5.0 (Windows NT 6.1; WOW64) SkypeUriPreview Preview/0.5
This isn't currently displayed in the Twitter cards despite it being in
the official documentation for cards, but other crawlers are definitely
using this metadata.