Comparison of the ACS grism spectra (data points with error bars) with ground-based spectra (gray lines). Upper panel: the source HAG_J033209.44-274806.8 identified as a broad absorption line QSO at z = 2.81 by Szokoloy et al. (2004) using VLT/FORS2 spectroscopy with the 150I grism (see Fig. 2 for the full preview image). Lower panel: the source HAG_J033239.83-275652.8, a spiral galaxy at z = 0.125, identified with a VLT/FORS2 spectrum taken with the 300I grism as part of the ESO-GOODS survey (Vanzella et al. 2008). The insets show the direct image stamps and, in the lower panel, also the 2D slitless stamp image.