We use one- and two-dimensional particle-in-cell simulations to demonstrate that the propagation of an ultraintense laser (I=10(19)W/cm(2)) in critical density plasma can be interfered with by a high density plasma wall region generated at the propagation front. When the electron flow speed of the wall region exceeds a certain relativistic threshold, the region behaves as an overdense plasma due to a decrease of the effective critical density. The region forms then very small overdense plasma islands. The islands impede the propagation intermittently and slow down the propagation speed significantly.