  The Wide Field Camera 3 is one of the instruments currently onboard the
Hubble Space Telescope and, since 2012, with the use of the spatial scanning
technique, has provided the largest number of observed exoplanetary atmosphere,
ranging from super-Earths to hot Jupiters. This technique enables the
observation of bright targets without saturating the sensitive detectors, but
at the same time requires more complicated data reduction and calibration
techniques to extract the planetary signal from the observations. In absence of
absolute calibration sources, the validation of current data analysis
techniques is not possible as the planetary signal is not known. Here, we
demonstrate how simulated observations can help us understand the effect of
different analysis processes, and potential unknown systematics on the final
transmission spectra of exoplanets. We test and validate the robustness of two
of the most precise WFC3 exoplanetary spectra - HD 209458 b and 55 Cancri e -
against three different known and potential sources of systematics. In
addition, we identify the horizontal shifts seen in WFC3 observations as the
most important source of systematic errors in the planetary spectra, concluding
that a precision better that 1% of a pixel is necessary.
