- Nexus 7 Android tablet: Settings > Apps > Chrome > Clear Cache
- Chrome (Mac OS): Settings (far right of address line) > Advanced Settings ... > Clear Browsing Data > Cached Images and Files
- Firefox (Mac OS): Preferences > Advanced > Network > Cached Web Content > Clear Now
You might try simply restarting your web browser or rebooting this computer. My iPhone was experiencing this problem and cleared up by itself before I could figure out how to clear the cache. So the good news is that this tends to be a self-fixing problem, but the time it might take to self-fix is quite unpredictable. Meanwhile, you have the YouTube versions.