Measurements of plasma density and electron temperature with Langmuir probes in low-density RF plasmas depend on proper compensation for oscillating potentials, but there are further difficulties. In regimes where Orbital-Motion-Limited (OML) theory should be valid, we find that the electron saturation current (Ie) cannot be obtained; instead, Ie rises almost linearly with probe voltage Vp. By monitoring the floating potential with a second probe, we find that a positive Vp on a cylindrical probe greatly affects the space potential Vs, dragging it up with Vp so that saturation is never obtained. This occurs even in a grounded chamber and is characteristic of all electrodeless discharges. The reason is that a large Ie to the probe has to be balanced by an equal ion current to the walls. To increase this ion current, an ambipolar potential has to be set up to push the ions outward against charge-exchange collisions. The drift in Vs to the new equilibrium state can take from milliseconds to seconds. Thus, a DC measurement gives erroneous results and must be corrected by the change in Vs. Another effect is an enhancement of the ion current at large negative Vp. This is probably due to the effect of ion-neutral collisions on the orbiting. Suggestions for improved probe techniques will be presented.