  NMR measurements have been performed on single crystals of
Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ (x = 0, 0.45) and CaFe$_2$As$_2$ grown from Sn
flux. The Ba-based pnictide crystals contain significant amounts of Sn in their
structure, $\sim 1$%, giving rise to magnetic impurity effects evident in the
NMR spectrum and in the magnetization. Our experiments show that the large
impurity magnetization is broadly distributed on a microscopic scale,
generating substantial magnetic field gradients. There is a concomitant 20%
reduction in the transition temperature which is most likely due to magnetic
electron scattering. We suggest that the relative robustness of
superconductivity ($x=0.45$) in the presence of severe magnetic inhomogeneity
might be accounted for by strong spatial correlations between impurities on the
coherence length scale.
