The Wide Field Camera 3 (WFC3) with its near-infrared capabilities on the Hubble Space Telescope has been a major milestone in finding high (z>9) redshift galaxies. Although a multitude of high redshift candidates were quickly identified, it has become clear that careful assumptions about the spectral energy distributions (SEDs) are required to exclude low redshift interlopers. Modern SED fitting to the broad band photometric measurements of these objects often cannot exclude low redshift solutions. Here, we present examples of such contamination and demonstrate that it is crucial to obtain spectroscopic confirmation of the redshifts of these sources. HST/WFC3, with its sensitive slitless spectroscopic mode has consistently proven itself to be a powerful instrument. We show through a case study of a z=11 CLASH candidate that even a modest investment in slitless spectroscopy can be used to effectively rule out low redshift interlopers.