On computations of angular momentum and its flux in numerical relativity
  The purpose of this note is to point out ambiguities that appear in the calculation of angular momentum and its radiated counterpart when some simple formulae are used to compute them. We illustrate, in two simple different examples, how incorrect results can be obtained with them. Additionally, we discuss the magnitude of possible errors in well known situations. 
Emanuel Gallo, Luis Lehner and Osvaldo M. Moreschi