  In BaFe2As2, structural and magnetic planar defects begin to proliferate
below the structural phase transition, affecting descriptions of magnetism and
superconductivity. We study using density-functional theory the stability and
magnetic properties of competing antiphase and domain boundaries, twins and
isolated $nano$twins (twin nuclei) - spin excitations proposed and/or observed.
These nanoscale defects have very low surface energy ($22$-$210$~$m$Jm$^{-2}$),
with twins favorable to the mesoscale. Defects exhibit smaller moments confined
near their boundaries -- making a uniform-moment picture inappropriate for
long-range magnetic order in real samples. {\it{Nano}}twins explain features in
measured pair distribution functions, so should be considered when analyzing
scattering data. All these defects can be weakly mobile and/or have
fluctuations that lower assessed "ordered" moments from longer spatial and/or
time averaging, and should be considered directly.
