Oh geez. This is a weird one.
Basically, I keep getting this.
I have no idea why. What I see in the Firefox status bar is an alternation between "Connecting to <an IP address>" and "Waiting for v<##>.lscache<#>.youtube.com" several times, before it displays the above message.
Only certain videos have this issue, and the affected videos keep changing. One day a video will be broken, and the next it'll be fine. Another video will be working, and the next time I try to view it, it's broken.
Here's the great bit:
Chrome and IE8 don't have this issue. This is only affecting my Firefox installation (3.6, by the way.)
I've tried clearing the cache, logging out and back in, removing the YouTube cookie, and even running it in Safe Mode (no add-ons, etc.), but no dice. Disabling AdBlock Plus, NoScript, or CookieSafe Lite does nothing.
What's going on?