  We report on quantum magneto-oscillations in an ultra-high mobility
GaAs/AlGaAs quantum well at very high tilt angles. Unlike previous studies, we
find that the spin and cyclotron splittings become equal over a continuous
range of angles, but only near certain, angle-dependent filling factors. At
high enough tilt angles, Shubnikov-de Haas oscillations reveal a prominent
beating pattern, indicative of consecutive level crossings, all occurring at
the same angle. We explain these unusual observations by an in-plane
field-induced increase of the carrier mass, which leads to accelerated, filling
factor-driven crossings of spin sublevels in tilted magnetic fields.
