I noticed that a few YouTube videos refused to play. Having searched around, I came upon a thread in the XBMC YouTube plugin code repository;
https://code.google.com/p/youtubexbmc/issues/detail?id=95#c217
martinpe...@gmail.com linked to an updated YouTubePlayer.py file;
Copying this file to your XBMC Addons directory, over the existing file, fixes the problem nicely.
Evidently, some YoutTube videos have a ‘$’ in the JS function name, which confuses the original plugin. This plugin replaces the ‘$’ in the function names with ‘_S_’
Hope this helps somebody!