When watching a YouTube video, the buffer bar can malfunction, causing the video to not load properly. It also makes it so the user cannot restart the video properly. This is a known problem and YouTube (hopefully) is working on it.
To solve the problem, you can do a few things: try clearing your web history and cache history. Make sure your browser (Internet Explore, Mozilla, Safari) is up to date. And make sure you have the latest Java software downloaded.
Source: http://www.google.com/support/youtube/bin/static.py?page=known_issues.cs
