  Intensive monitoring campaigns have recently attempted to measure the time
delays between multiple images of gravitational lenses. Some of the resulting
light-curves show puzzling low-level, rapid variability which is unique to
individual images, superimposed on top of (and concurrent with) longer
time-scale intrinsic quasar variations which repeat in all images. We
demonstrate that both the amplitude and variability time-scale of the rapid
light-curve anomalies, as well as the correlation observed between intrinsic
and microlensed variability, are naturally explained by stellar microlensing of
a smooth accretion disk which is occulted by optically-thick broad-line clouds.
The rapid time-scale is caused by the high velocities of the clouds (~5x10^3
km/s), and the low amplitude results from the large number of clouds covering
the magnified or demagnified parts of the disk. The observed amplitudes of
variations in specific lenses implies that the number of broad-line clouds that
cover ~10% of the quasar sky is ~10^5 per 4 pi steradian. This is comparable to
the expected number of broad line clouds in models where the clouds originate
from bloated stars.
