The algorithm for post-processing of return value requires that:

1. If result is:
    (...)
    5. an object, then return the object created by recursively applying this algorithm to each property in result. 


If the object is self-recursive (i.e. var x = {}; x['parent'] = x), this causes an infinite recursion. Such objects are actually more common then you would expects. For example, the popular library jQuery represents it's search results as objects that are self-recursive.