  Recently, a fast radio burst (FRB)-like event is found to be associated with
a Galactic magnetar, SGR 1935+2154, accompanied by an X-ray burst. We find this
radio burst challenges the typical emission mechanisms involving magnetars,
which includes coherent curvature radiation from charged bunches, fast
magnetosonic (FMS) wave, synchrotron maser from shocks, and the pulsar-like
mechanism for low-twist magnetars. More specifically, we find that (1) the
X-rays are most-likely to be produced inside the magnetosphere. (2) For the
coherent curvature radiation from the decay of Alfv\'en wave, it will generally
predict a duration ($\lesssim0.1$ ms) smaller than observations, because of the
strong twists of magnetic field lines and the internal damping of Alfv\'en
waves. (3) The FMS wave model predicts a very low emission frequency $\nu_{\rm
p}\sim0.03$ MHz $\ll$ GHz, unless it is produced inside the magnetosphere. But
the absorption effect of the magnetospheric FMS wave model remains to be
studied. (4) The synchrotron maser model is challenged, because observations
show that the peaks in both X-ray and radio light curves are with the same
temporal separation $\Delta t_{\rm FRB}=\Delta t_\gamma\approx0.03$ s, while it
would predict $\Delta t_{\rm FRB}\ll\Delta t_\gamma$. (5) It seems to be
difficult to directly apply the low-twist pulsar-like mechanism to flaring
magnetars, as magnetar activity can significantly deform the magnetosphere. (6)
We suggested four possibilities to study the general properties of FRBs for
future observations, especially the possibility of identifying quasi-periodic
oscillations with period $\sim1-10$ ms in double/multiple-peaked FRBs.
