<jats:title>Abstract</jats:title><jats:p>We investigate the accuracy with which the reconnection electric field <jats:italic>E</jats:italic><jats:sub><jats:italic>M</jats:italic></jats:sub> can be determined from in situ plasma data. We study the magnetotail electron diffusion region observed by National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) on 11 July 2017 at 22:34 UT and focus on the very large errors in <jats:italic>E</jats:italic><jats:sub><jats:italic>M</jats:italic></jats:sub> that result from errors in an <jats:italic>L</jats:italic><jats:italic>M</jats:italic><jats:italic>N</jats:italic> boundary normal coordinate system. We determine several <jats:italic>L</jats:italic><jats:italic>M</jats:italic><jats:italic>N</jats:italic> coordinates for this MMS event using several different methods. We use these <jats:italic>M</jats:italic> axes to estimate <jats:italic>E</jats:italic><jats:sub><jats:italic>M</jats:italic></jats:sub>. We find some consensus that the reconnection rate was roughly <jats:italic>E</jats:italic><jats:sub><jats:italic>M</jats:italic></jats:sub> = 3.2 ± 0.6 mV/m, which corresponds to a normalized reconnection rate of 0.18 ± 0.035. Minimum variance analysis of the electron velocity (MVA‐<jats:italic>v</jats:italic><jats:sub><jats:italic>e</jats:italic></jats:sub>), MVA of <jats:italic>E</jats:italic>, minimization of Faraday residue, and an adjusted version of the maximum directional derivative of the magnetic field (MDD‐<jats:italic>B</jats:italic>) technique all produce reasonably similar coordinate axes. We use virtual MMS data from a particle‐in‐cell simulation of this event to estimate the errors in the coordinate axes and reconnection rate associated with MVA‐<jats:italic>v</jats:italic><jats:sub><jats:italic>e</jats:italic></jats:sub> and MDD‐<jats:italic>B</jats:italic>. The <jats:italic>L</jats:italic> and <jats:italic>M</jats:italic> directions are most reliably determined by MVA‐<jats:italic>v</jats:italic><jats:sub><jats:italic>e</jats:italic></jats:sub> when the spacecraft observes a clear electron jet reversal. When the magnetic field data have errors as small as 0.5% of the background field strength, the <jats:italic>M</jats:italic> direction obtained by MDD‐<jats:italic>B</jats:italic> technique may be off by as much as 35°. The normal direction is most accurately obtained by MDD‐<jats:italic>B</jats:italic>. Overall, we find that these techniques were able to identify <jats:italic>E</jats:italic><jats:sub><jats:italic>M</jats:italic></jats:sub> from the virtual data within error bars ≥20%.</jats:p>