  We report the analysis of the first resolved caustic-crossing binary-source
microlensing event OGLE-2016-BLG-1003. The event is densely covered by the
round-the-clock observations of three surveys. The light curve is characterized
by two nested caustic-crossing features, which is unusual for typical
caustic-crossing perturbations. From the modeling of the light curve, we find
that the anomaly is produced by a binary source passing over a caustic formed
by a binary lens. The result proves the importance of high-cadence and
continuous observations, and the capability of second-generation microlensing
experiments to identify such complex perturbations that are previously unknown.
However, the result also raises the issues of the limitations of current
analysis techniques for understanding lens systems beyond two masses and of
determining the appropriate multiband observing strategy of survey experiments.
