  In order to develop and test a methodology to search for UV variability over
the entire GALEX database down to the shortest time scales, we analyzed
time-domain photometry of $\sim5000$ light curves of $\sim300$ bright
$(m_{\rm{FUV}}, m_{\rm{NUV}} \leq 14)$ and blue $(m_{\rm{FUV}} - m_{\rm{NUV}} <
0)$ GALEX sources. Using the \gphoton \ database tool, we discovered and
characterized instrumentally-induced variabilities in time-resolved GALEX
photometry, which may severely impact automated searches for short-period
variations. The most notable artifact is a quasi-sinusoidal variation mimicking
light curves typical of pulsators, seen occasionally in either one or both
detectors, with amplitudes of up to 0.3 mag and periods corresponding to the
periodicity of the spiral dithering pattern used during the observation
(P$\sim$120 sec). Therefore, the artifact may arise from small-scale response
variations. Other artifacts include visit-long "sagging" or "hump" in flux,
occurring when the dithering pattern is not a spiral, or a one-time change in
flux level during the exposure. These instrumentally-caused variations were not
reported before, and are not due to known (and flagged) artifacts such as hot
spots, which can be easily eliminated. To characterize the frequency and
causality of such artifacts, we apply Fourier transform analysis to both light
curves and dithering patterns, and examine whether artificial brightness
variations correlate with visit or instrumental parameters. Artifacts do not
correlate with source position on the detector. We suggest methods to identify
artifact variations and to correct them when possible.
