3,3,3-Trifluoropropene, $\mathrm{CF}{}_3-\mathrm{CH}=\mathrm{CH}{}_2$, reacts with HBr in the dark. The major product is

The strongly electron-withdrawing $\mathrm{CF}{}_3$ group destabilises an adjacent positive charge. Protonation at the internal carbon would put positive charge next to $\mathrm{CF}{}_3$. Instead H${}^{+}$ adds to the carbon bearing $\mathrm{CF}{}_3$, giving the primary cation $\mathrm{CF}{}_3-\mathrm{CH}{}_2-\mathrm{CH}{}_2{}^{+}$ that is farther from the $\mathrm{CF}{}_3$. The product is $\mathrm{CF}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{Br}$ (apparent anti-Markovnikov).