The study of exoplanetary atmospheres with low-resolution transmission spectroscopy relies on measuring minute changes in the transit depth with wavelength and a number of ground- and space-based instruments have been used to characterize exoplanets in different spectral bands. For the last decade, these instruments have each only probed a narrow spectral region, which has motivated the community to combine observations from different instruments in order to achieve a broader wavelength coverage. By analysing Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) data of KELT-11 b, we once again show the risks of following this now conventional approach. We demonstrate that changes in the reduction or analysis method can lead to drastic differences in the mean transit depth and that combining this with additional data can lead to discrepant interpretations of the atmospheric composition. With the launch of JWST, and its many available instruments and modes, observers may be tempted to combine data sets at longer wavelengths (e.g. NIRSpec – Near Infrared Spectrometer) with those from HST STIS (Space Telescope Imaging Spectrograph) or WFC3 without the consideration of offsets or other incompatibilities. Given the obvious potential issues, we caution against such an approach and encourage the community to thoroughly address the issue of data incompatibility instead of adhering to a de facto assumption of compatibility.
