  In this paper we compare experimental observations and theory of radiation
emission from a microbunched beam with microbunching wavefront tilt with
respect to the direction of motion. The theory refers to the work [1], which
predicts, in this case, exponential suppression of coherent radiation along the
kicked direction. The observations refer to a recent experiment performed at
the LCLS [2,3], where a microbunched beam was kicked by a bend and sent to a
radiator undulator. The experiment resulted in the emission of strong coherent
radiation that had its maximum along the kicked direction of motion, when the
undulator parameter was detuned to a value larger than the nominal one. We
first analyze the theory in detail, and we confirm the correctness of its
derivation according to the conventional theory of radiation emission from
charged particles. Subsequently, we look for possible peculiarities in the
experiment, which may not be modeled by the theory. We show that only spurious
effects are not accounted for. We conclude that the experiment defies
explanation in terms of the conventional theory of radiation emission.
