  We use the rapid binary population synthesis code COMPAS to investigate
commonly used prescriptions for the determination of mass transfer stability in
close binaries and the orbital separations after stable mass transfer. The
degree of orbital tightening during non-conservative mass transfer episodes is
governed by the poorly-constrained angular momentum carried away by the ejected
material. Increased orbital tightening drives systems towards unstable mass
transfer leading to a common envelope. We find that the fraction of interacting
binaries that will undergo only stable mass transfer throughout their lives
fluctuates between a few and $\sim 20\%$ due to uncertainty in the angular
momentum loss alone. If mass transfer is significantly non-conservative,
stability prescriptions that rely on the assumption of conservative mass
transfer under-predict the number of systems which experience unstable mass
transfer and stellar mergers. This may substantially impact predictions about
the rates of various transients, including luminous red novae,
stripped-envelope supernovae, X-ray binaries, and the progenitors of coalescing
compact binaries.
