While attempting to form tantalum-based shallow ohmic contacts to the two-dimensional electron gas (2DEG) confined in GaN/AlGaN heterostructures, it was noticed that a parasitic channel in a few hundred nanometers depth is contacted and exhibits ohmic behavior after contact metal stack deposition and before additional thermal annealing. This indicates significant metal diffusion into the GaN/AlGaN stack and prevents the separation of the individual contributions of the 2DEG and the parasitic channel to the lateral electrical conductivity. Post-deposition thermal treatment only slightly increases electrical conductivity, which is clearly dominated by the parasitic channel. In contrast, conductivity in a reference GaN/AlGaN stack with a 2DEG as the only conductive channel increases by a few orders of magnitude after the ohmic contact is formed upon thermal annealing. Element specific analysis confirms high concentrations and, thus, sufficient diffusion of contact stack metals to a parasitic layer several hundred nanometers into GaN. Our findings demonstrate the difficulties in shallow ohmic contact formation for GaN/AlGaN stacks, which might become of interest in GaN-on-GaN high-frequency applications.