A set of wave data generated in an offshore tank is studied. The set of the largest waves in those sea states is studied with respect to steepness, asymmetry, and nonlinearity. It was possible to find some waves that were classified as abnormal or freak according to presently accepted criteria. Their statistics connected to steepness and nonlinearity was determined. Comparison of the nonlinearity as well as statistics of the largest waves in the offshore tank and at sea was performed so as to identify any limitation in reproduction of the properties of large waves in model basins.