  We report the observation and characterisation of a systematic error in the
implementation of $U_3$ gates in the IBM quantum computers. The error appears
as a consistent shift in one of the angles of the gate, whose magnitude does
not correlate with IBM's cited errors calculated using Clifford randomized
benchmarking. We propose a simple mitigation procedure, leading to an
improvement in the observed value for the CHSH inequality, highlighting the
utility of simple mitigation strategies for short-depth quantum circuits.
