There is no doubt that computational chemistry can aid in the development and comprehension of reactions and molecules. In particular, it is possible to understand the stability of chemical systems by obtaining their decomposition mechanisms and simply calculating the degradation rate and half-life from the pathway with the lowest activation energy. However, due to the structural strain of many "fleeting"molecules, it is possible that they would be completely unsynthesizable thanks to the tunnelling effect, which acts even at the lowest temperatures. If that is the case, then computational quantum chemistry can predict whether trying to synthesize a hypothetical molecule is a worthwhile goal or if it is like fighting against windmills. Through some discussions and examples, we will use the quantum tunnelling instability concept as an excuse to figure out reactions driven by heavy-atom tunnelling.