function abc() {
    var a = 1;
    var func = function() {
        var a = 2;
    }
    func();
    alert(a);
}

Pay attention to the var, in the piece of code, the result of a will be 1, but if the var is omitted, the result will be 2, but I found Coffee not able to translate to this.

For example the following:

abc = ->
    a = 1
    func = ->
        a = 2
        return
    func()
    alert(a)
    return