  The application of Wilson's Numerical Renormalization Group (NRG) method to
dissipative quantum impurity models, in particular the sub-ohmic spin-boson
model, has led to conclusions regarding the quantum critical behavior which are
in disagreement with those from other methods and which are by now recognized
as erroneous. The errors of NRG remained initially undetected because NRG
delivered an internally consistent set of critical exponents satisfying
hyperscaling. Here we discuss how the conspiracy of two errors - the
Hilbert-space truncation error and the mass-flow error - could lead to this
consistent set of exponents. Remarkably, both errors, albeit of different
origin, force the system to obey naive scaling laws even when the physical
model violates naive scaling. In particular, we show that a combination of the
Hilbert-space truncation and mass-flow errors induce an artificial non-analytic
term in the Landau expansion of the free energy which dominates the critical
behavior for bath exponents s<1/2.
