In our experiments designed to synthesize cyclocholate derivatives by the Yamaguchi macrocyclization in commercial THF solvent, mysterious needles were isolated by column chromatography. Apparently these crystals had a significant vapor pressure because they tended to grow into long needles within a closed container over time. The FAB mass spectrum did not resolve their identity, but X-ray crystallography showed their identity to be 2,6-di-tert-butyl-4-methyl phenol. Its origin is from the commercial anhydrous THF which uses BHT (butylated hydroxytoluene) at the 250 ppm level as an inhibitor to peroxide formation. The caveat: use inhibitor-free THF as a solvent in Yamaguchi and other reactions.