Already at coverages as low as 0.005 monolayer of potassium on room temperature Si(111)7×7 surfaces, atomic resolution is gradually lost when imaging the surface with scanning tunneling microscope at positive sample bias, giving rise to bright triangles over the faulted halves of the 7×7 unit cells and dark triangles above the unfaulted halves. We suggest that this is due to potassium atoms that are picked up by the tip fromKislands at the surface, as is evident by the observed lowering of the tunneling energy barrier. This phenomenon vanishes upon thermal desorption ofKislands or upon exposure to 0.1 L oxygen.