Trans-hydrogenation of alkenes
This link has a good summary of hydrogenation. Syn addition is very much preferred because of the way the molecule 'adheres' to the surface. As you note, however, there are cases where anti-addition is observed - particularly for sterically hindered molecules such as 1,2-dimethylcyclohex-1-ene which shows up to 30% of the trans/anti-addition product (which also so happens to be the thermodynamic product).

The reason we see the syn-addition more often is because the Hydrogen atom attack happens really quickly (it's the kinetic product). If you pump more heat energy into the reaction though, you could expect to see more trans product - but this is dangerous as hydrogen is explosive.