  We present experimental evidence for a hitherto unconfirmed type of
angle-dependent magnetoresistance oscillation caused by magnetic breakdown. The
effect was observed in the organic superconductor
kappa-(BEDT-TTF)$_2$Cu(NCS)$_2$ using hydrostatic pressures of up to 9.8 kbar
and magnetic fields of up to 33 T. In addition, we show that similar
oscillations are revealed in ambient pressure measurements, provided that the
Shubnikov-de Haas oscillations are suppressed either by elevated temperatures
or filtering of the data. These results provide a compelling validation of
Pippard's semiclassical picture of magnetic breakdown.
