The collectionType attribute of h:selectManyCheckbox is not processed correctly.
The value attribute of selectManyCheckbox points to a Map with an ArrayList containing Strings as values, representing the selected values.
The checkboxes are initially correctly checked, but after ajax submission the model (Map<Sting,Object) contains StringArrays instead of ArrayLists.
This worked well since JSF 1.0, but it does not work with 2.1 anymore.
It can be easily reproduced with MyFaces only.
I see this as critical because ClassCastException are thrown as a result in application code.