I embed e.g. Vimeo-Videos by pasting an URL like
into my post. That gets transformed into an embedded video as expected.
BUT: In my RSS-feed it does not work, as the iFrame's URL reads
My feed reader (Fever) interprets this as a local URL to my blog, which leads to a 404. BTW: I see this a lot, even on other blogs and/or previous versions of wordpress