As massive binary companions age they will swell to dimensions comparable to the distance between stars. The larger star will likely begin transferring mass to its companion, which will accrete mass and angular momentum and may spin up to near critical velocity. Additional mass transfer may lead to large scale mass loss. Here, we present an observational study of three systems that are experiencing or are products of mass transfer. VFTS~102 and VFTS~285 are the fastest rotating stars currently known. We present an analysis of their far-ultraviolet and visible band spectral features and determine their physical properties and angular momentum loss processes. We conclude that VFTS~102 is the result of a stellar merger and that VFTS~285 was ejected when its companion exploded in a supernova. Lastly, the interacting binary W~Serpentis is emerging from its most intense phase of mass transfer. W~Serpentis is an eclipsing binary and the prototype of the Serpentid class of interacting binaries. We present an observational analysis of high quality, visible band spectroscopy and the first near-IR, long-baseline interferometric observations. We present examples of the appearance and radial velocities of the primary spectral components: prominent emission lines, strong shell lines, and weak absorption lines. We find some of the weak absorption features are associated with the cool mass donor, and we present the first radial velocity curve for the donor star. The donor’s absorption lines are rotationally broadened, and we derive a mass ratio of 0.49 +\- 0.07. We also derive the orbital inclination and mass estimates of 3.5 M_⊙ and 7.2 M_⊙ for the donor and gainer, respectively. The partially resolved interferometric measurements are consistent with our derived orbital properties and the Gaia EDR3 distance. Spectroscopic evidence indicates that the gainer is obscured by an opaque disk that channels the mass transfer stream into an outflow through the L3 region and out into a circumbinary disk. The results demonstrate that binary stars may lose a significant fraction of their mass during the fleeting but transformative stage of mass transfer.