There are two versions of Yahoo! Media Player at any given time.
One is at http://mediaplayer.yahoo.com/js . This version is the most reliable one. It is for production sites which can live without the latest features and can't live with a buggy user experience.
One is at http://mediaplayer.yahoo.com/latest . This version has the newest features. It is for developer sites which can live with bugs and need the latest features.
The version at /js is conservative. The version at /latest is bleeding edge: it is *guaranteed to be buggy.* Sometimes it is completely unusable. If you use it on a public site, you would be wise to set user expectations.