C(CH₃)₅⁺, a pentacoordinate carbocation, was recently described as a “fleeting” molecule, that is, stable only at low temperatures. Herein it is shown using theoretical methods that, owing to carbon tunneling, the molecule is not persistent even at 0 K. One possible way to lengthen its lifespan is by deuterium substitution, in spite of the fact that no bond to hydrogen is broken.