I experience this annoying effect and I don't understand if the problem is of the OS (OS X), the browser (Chrome), the Flash player, or YouTube itself. Everything is upgraded to the latest version, with the exception of OS X (10.6.8).
What happens is the following: when a movie is playing, the moving circle cursor indicating the position is always followed (or preceded, your choice) by a light grey area of buffering. I can freely move the cursor within this area by clicking in any position, and the movie will correctly go to that position, but if I try to click to an area that is still not buffered (dark grey), the cursor will move there for a brief interval and then immediately bounce back to the end of the available buffer (the end of the grey area). Occasionally, if I either drag the cursor instead of clicking, or if I pause the movie and then move the cursor, it will behave as I expect (starting the movie from the position I choose) but this is rare.
Am I the only one experiencing this? How can I fix it?