Artifacts in high resolution HAADF STEM imaging can be induced if the specimen is too thick. For instance, Figure 2936 shows high resolution STEM ADF images taken from (001) SrTiO3 single crystals in different specimen thicknesses (t). The ADF lattice periodicity of the specimen in the thickness of t/λ=6 does not represent the periodicity of the crystal potential, indicating there is an imaging artifact when the specimen is too thick.
Figure 2936. High resolution STEM ADF images taken from (001) SrTiO3 single crystals as a function of crystal thickness: (a) t/λ = 0.25, (b) =0.5, (c) = 0.9, (d) = 1.2, (e) =6. λ is the mean free path (λ = ~115 nm for SrTiO3) and ADF collection angle here was 46–104 mrad.
Adapted from 
It should be highlighted that the ADF contrast maximum did not occur at the thinnest area but at t/λ = 0.9. They  suggested the reason for that is there was a surface amorphous layer on the specimen. Therefore, in very thin regions, the fraction of the crystal is small comparing with that of amorphous surface, resulting in lower image contrast.
If the sample is more than a few nm thick, the smooth region will be replaced by faint random mist because the top and bottom cannot be at infinite magnification at the same defocus. 
 H. Inada, D.Su, R. F. Egerton, M.Konno, L.Wu, J.Ciston, J.Wall, Y.Zhu, Atomic imaging using secondary electrons in a scanning transmission electron microscope: Experimental observations and possible mechanisms, Ultramicroscopy 111(2011)865–876.
 M. Weyland and D. A. Muller, Tuning the convergence angle for optimum STEM performance, FEI nanoSolutions, (2006) 24.