It is possible for unreported Global Positioning System satellite faults to cause phase variations mimicking the effect of ionospheric scintillation. A case study of an event on 17 May, 2011 is presented. For approximately 695 s, the L1 signal from the Navstar 43 satellite (pseudo-random number 13) contained pulses of rapid phase variation, in such a manner as to cause a large rise in the sigma-phi scintillation metric. The event was simultaneously observed from two receivers in England, placed 190 km apart. A range of other explanations, that included genuine ionospheric scintillation, were considered but found to be highly unlikely. We therefore recommend that precautions be taken when interpreting phase scintillation values, to prevent satellite faults from contaminating data.