  In underdoped YBa$_2$Cu$_3$O$_{6+x}$, there is evidence of a small Fermi
surface pocket subject to substantial mass enhancement in the doping regime $
0.12<p<0.16$. This mass enhancement may vary substantially over the Fermi
surface, due to "hot spot" or other relevant physics. We therefore examine the
magnetotransport of an electron-like Fermi pocket with large effective mass
anisotropy. Within the relaxation time approximation, we show that even for a
pocket with a fixed shape, the magnitude and sign of the Hall effect may change
as the mass anisotropy changes (except at very large, likely inaccessible
magnetic fields). We discuss implications for recent Hall measurements in near
optimally doped cuprates in high fields. In addition we identify a novel
intermediate asymptotic regime of magnetic field, characterized by B-linear
magnetoresistance. Similar phenomena should occur in a variety of other
experimental systems with anisotropic mass enhancement
