Reactions involving the transition between electronic states of different spin, often called intersystem crossing or spin crossovers, are widespread throughout nature. As an essential process of photochemistry, they play key roles in various scientific fields such as organic light-emitting diodes, 8 synthetic chemistry, controlling magnetic properties of materials, and photodynamic therapy of cancer and other skin diseases. Despite their prevalence, they have somewhat of a special standing in quantum chemistry since they cannot be captured by the most common work equations. Spin crossovers involve multiple electronic states and thus belong to the class of electronically nonadiabatic reactions, for which the Born-Oppenheimer approximation is not valid. On an even more fundamental level, the (nonrelativistic) Schrödinger equation decouples the electron's spin from its orbital motion, which would imply that transitions between states of different spin are forbidden. In reality, however, such transitions, though typi-cally much slower than spin-allowed processes, are possible and are mainly facilitated by the relativistic spin-orbit coupling (SOC). Received wisdom suggests that quantum tunneling effects will only be significant for reactions involving hydrogen atoms and that useful tunneling corrections can be devised based on the shape of a one-dimensional cut through the barrier. However, this is based on experience with adiabatic reactions taking place on single Born-Oppenheimer surfaces, which typically exhibit relatively broad potential barriers with rounded tops. In contrast, electronically nonadiabatic reactions such as spin crossovers, particularly those in the Marcus inverted regime, feature narrow, cusped barriers, and it has been proposed that tunneling may play a more important role. This has recently been confirmed by a set of studies of carbenes and nitrenes in matrix isolation at cryogenic temperatures. Yet as we shall demonstrate, strong tunneling effects are not limited to such extreme environments but can also play a crucial role at room temperature in these nonadiabatic reactions and even allow for heavy-atom tunneling. Importantly, we find that the "corner cutting" effects first described in the context of adiabatic reactions are much more important for nonadiabatic reactions and can drastically change the reaction mechanism. This means that the rate cannot be calculated with a simple one-dimensional tunneling correction, but that it is essential to take account of multidimensional heavy-atom tunneling in the reaction mechanism.