Two anomalous effects seen in the simulation of partially depleted SOI MOSFETs are reported. The first is an unrealistic steep increase in the drain current due to impact ionization, predicted by both the drift-diffusion (DD) and energy transport (ET) models. Next, in ET simulations a negative differential output conductance is observed. The latter effect is neither present in DD simulations nor is it known from measurements. A comprehensive simulation study reveals that the effect is caused by an overestimation of hot carrier diffusion into the floating body.