Update:

Instead of using

• $("#mySelector:selected") or
• $("#mySelector:checked")

Use

• $("#mySelector").filter(":selected") or
• $("#mySelector").filter(":checked")

Using $("...").is(":selected") or $("...").is(":checked") works fine as well.

Reason:
If you change the selected <option>, the :selected selector still brings the initially selected one.

http://jsbin.com/nematiteba/1/edit?html,js,output

:checked has problems as well.