cis-But-2-ene is converted to butane-2,3-diol by two routes: (I) mCPBA, then dilute aqueous $\mathrm{H}{}_3\mathrm{O}{}^{+}$; (II) cold, dilute, alkaline $\mathrm{KMnO}{}_4$. The stereochemical outcomes are

Route I: syn epoxidation gives the cis epoxide, and anti opening by water then gives the (2R,3R) and (2S,3S) diols in equal amounts (racemic). Route II: syn dihydroxylation of the cis alkene gives the meso (2R,3S) diol. The same alkene thus gives opposite diastereomers by the two routes.