Details of one CARMENES spectrum of GJ 1214 around the He I triplet lines. A raw spectrum after standard data reduction is plotted in green. The same spectrum after the telluric correction with molecfit is over-plotted in orange. The same spectrum after removing the OH emission features is over-plotted in black. The red shaded region is the spectral range used to normalise the continuum of all spectra. The vertical blue dashed lines indicate the position at mid-transit of He I triplet lines and the blue shaded regions represent the planet traces during the transit. All the spectra are shifted to the stellar rest frame at vacuum wavelength.