There are probably a several good ways to solve the following problem:
1. you have a simple select list which cascades a shuttle element
2. you select something and the shuttle is populated
3. after that, you pick a value in the shuttle and move it to the rigt side
4. then you change the select list again
5. the values you selected in your shuttle are now lost
This example shows how this can be done. This question apears in the APEX Forum from time to time and I decided to provide one simple solution for it.