  The study of exoplanetary atmospheres relies on detecting minute changes in
the transit depth at different wavelengths. To date, a number of ground and
space based instruments have been used to obtain transmission spectra of
exoplanets in different spectral band. One common practice is to combine
observations from different instruments in order to achieve a broader
wavelength coverage. We present here two inconsistent observations of WASP-96
b, one by Hubble Space Telescope (HST) and the other by the Very Large
Telescope (VLT). We present two key findings in our investigation: 1.) a strong
water signature is detected via the HST WFC3 observations. 2.) A notable offset
in transit depth (> 1100 ppm) can be seen when the ground-based and space-based
observations are combined together. The discrepancy raises the question of
whether observations from different instruments could indeed be combined
together. We attempt to align the observations by including an additional
parameter in our retrieval studies but are unable to definitively ascertain
that the aligned observations are indeed compatible. The case of WASP-96 b
signals that compatibility of instruments should not be assumed. While
wavelength overlaps between instruments can help, it should be noted that
combining datasets remains a risky business. The difficulty in combining
observations also strengthens the need for next generation instruments which
will possess broader spectral coverage.
