A coworker of mine got stung by this last week. While refactoring some logic he needed to reverse a result of a checkbox checked. Knowing that .is() returns a boolean he assumed that .not() would return the inverse.
This is what he was trying to get working, but as you will read on it did not work as expected.
Per the JQuery .not() documentation the value returned into the notChecked variable was actually a jquery object containing a subset of elements which do not match the checked rule and not a simple boolean.