An alkene A on treatment with mCPBA followed by dilute aqueous acid gives a meso diol, while the same alkene with $\mathrm{OsO}{}_4$ followed by $\mathrm{NaHSO}{}_3$ gives a racemic diol. A is

mCPBA/H3O+ results in net anti addition of two OH groups, and OsO4 in syn addition. For trans-but-2-ene anti addition gives the meso diol and syn addition gives the racemic pair. For cis-but-2-ene the outcomes are reversed, and but-1-ene or 2-methylpropene cannot give a meso diol.