  We report a stellar flare occurring during a transit of the exoplanet
OGLE-TR-10b, an event not previously reported in the literature. This reduces
the observed transit depth, particularly in the u'-band, but flaring could also
be significant in other bands and could lead to incorrect planetary parameters.
We suggest that OGLE-TR-10a is an active planet-hosting star and has an
unusually high X-ray luminosity.
