  We report the discovery of a transient source in the central regions of
galaxy cluster Abell 267. The object, which we call "PALS1", was found in a
survey aimed at identifying highly-magnified Lyman-break galaxies in the fields
of intervening rich clusters. At discovery, the source had U>24.7 (2-sigma;
AB), g=21.96, and very blue g-r and r-i colors; i.e., PALS1 was a U-band
drop-out, characteristic of star-forming galaxies and quasars at z~3. However,
three months later the source had faded by more than three magnitudes. Further
observations showed a continued decline in luminosity, to R>26.4 seven months
after discovery. Though the apparent brightness is suggestive of a supernova at
roughly the cluster redshift, we show that the photometry and light curve argue
against any known type of supernova at any redshift. The spectral energy
distribution and location near the center of a galaxy cluster are consistent
with the hypothesis that PALS1 is a gravitationally-lensed transient at z~3.3.
If this interpretation is correct, the source is magnified by a factor of 4-7
and two counterimages are predicted. Our lens model predicts time delays
between the three images of 1-10 years and that we have witnessed the final
occurrence of the transient. The intense luminosity (M(AB) ~ -23.5 after
correcting for lensing) and blue UV continuum (implying T>50,000 K) argue the
source may have been a flare resulting from the tidal disruption of a star by a
10^6-10^8 solar-mass black hole. Regardless of its physical nature, PALS1
highlights the importance of monitoring regions of high magnification in galaxy
clusters for distant, time-varying phenomena.
