The theoretical modeling of massive star evolution is riddled with uncertainties. Two such unresolved aspects are mass loss by stellar winds and mixing in stellar interiors. In my talk I will discuss the implications of these uncertainties on reconciling theory with observations of massive stellar endpoints. In the first part I will present the role of winds for progenitors of stripped-envelope supernovae. While it is commonly assumed that binary interaction removes the hydrogen envelope, the assumed post-interaction mass-loss rate is crucial for the resulting supernova type. I will show how we assess the hydrogen mass in stripped-envelope supernova progenitors by comparing pre-supernova photometry with evolution models. In the second part of the talk I will focus on the lack of observed cool and very luminous supergiant stars, an unexplained phenomenon termed the “Humphreys-Davidson Limit”, which is in contrast to simulations of evolved massive stars. I will show how enhanced mixing above convective cores can aid in removing the hydrogen envelope and ease the tension between theory and observations for such stars at the high-mass end, which are also thought to be the progenitors of merging black holes.