We had this Yahoo! Pool glitch before for a short time period and i am sure most Yahoo! pool players was unaware about it.
When you begin the game everything looks normal but during the game yellow solid ball suddenly get lost from the table.We discovered it at 12/04/2006 and hope Yahoo! games team fix this glitch in a short time again.
You can see a screenshot about this glitch at Yahoo! Pool glitches page.