Factor XII (FXII) deficiency is a coagulation factor disorder inherited in an autosomal recessive manner that causes prolonged activated partial thromboplastin time (aPTT). Although it is clinically benign without additional bleeding risk, it has been associated with paradoxical thrombosis. We report the case of a 38-year-old woman who presented with cardiac arrest secondary to spontaneous coronary artery dissection (SCAD) in the absence of identifiable risk factors. Laboratory investigations revealed an isolated prolonged aPTT, which was corrected with mixing studies, and a severe FXII deficiency with activity levels < 1%. Molecular analysis identified two homozygous FXII gene variants: the 46C>T polymorphism and the c.619G>C (p.Ala207Pro) variant. The two variants are considered clinically benign, although the combination of both in a single individual has not been previously reported. The coexistence of FXII deficiency in this case may be of clinical and hypothesis-generating interest.