  Numerically exact path-integral Monte Carlo data are presented for $N\leq 10$
strongly interacting electrons confined in a 2D parabolic quantum dot,
including a defect to break rotational symmetry. Low densities are studied,
where an incipient Wigner molecule forms. A single impurity is found to cause
drastic effects: (1) The standard shell-filling sequence with magic numbers
$N=4,6,9$, corresponding to peaks in the addition energy $\Delta(N)$, is
destroyed, with a new peak at N=8, (2) spin gaps decrease,
  (3) for N=8, sub-Hund's rule spin S=0 is induced, and (4) spatial ordering of
the electrons becomes rather sensitive to spin. We also comment on the recently
observed bunching phenomenon.
