Your eyes may fool you...
Keep your differential diagnosis open.
Selected References
Aravindhan N, Chisholm DG. Sulfhemoglobinemia presenting as pulse oximetry desaturations. Anesthesiology. 2000;93:883–884.
So T-Y et al. Topical Benzocaine-induced Methemoglobinemia in the Pediatric Population. J Pediatr Health Care. 22(6):335–339.