Equatorial ionospheric scintillation data at 1541.5 Mhz (L-band) and 3945.5 MHz (C-band) showing time shifts of up to one second between similar fades of the two signals are presented. Simple model computations show that systematic refractive effects due to equatorial plasma bubbles in the particular propagation geometry may explain the observed data. Implications on equatorial ionospheric irregularities and scintillation theory are discussed.