  Transmission spectroscopy is a promising tool for the atmospheric
characterization of transiting exoplanets. Because the planetary signal is
faint, discrepancies have been reported regarding individual targets. We
investigate the dependence of the estimated transmission spectrum on deviations
of the orbital parameters of the star-planet system that are due to the
limb-darkening effects of the host star. We describe how the uncertainty on the
orbital parameters translates into an uncertainty on the planetary spectral
slope. We created synthetic transit light curves in seven different wavelength
bands, from the near-ultraviolet to the near-infrared, and fit them with
transit models parameterized by fixed deviating values of the impact parameter
$b$. Our simulations show a wavelength-dependent offset that is more pronounced
at the blue wavelengths where the limb-darkening effect is stronger. This
offset introduces a slope in the planetary transmission spectrum that becomes
steeper with increasing $b$ values. Variations of $b$ by positive or negative
values within its uncertainty interval introduce positive or negative slopes,
thus the formation of an error envelope. The amplitude from blue optical to
near-infrared wavelength for a typical uncertainty on $b$ corresponds to one
atmospheric pressure scale height and more. This impact parameter degeneracy is
confirmed for different host types; K stars present prominently steeper slopes,
while M stars indicate features at the blue wavelengths. We demonstrate that
transmission spectra can be hard to interpret, basically because of the
limitations in defining a precise impact parameter value for a transiting
exoplanet. This consequently limits a characterization of its atmosphere.
