This paper examines the inefficiency issues of occupation-specific human capital investments in a model with search frictions and occupational mobility over the life cycle. Firstly, we highlight how the existence of social externalities related to training, not taken into account by employers, cause an under-investment in occupation-specific vocational training compared with what it would be optimal to do. Secondly, we show that this lack of training leads to some occupational mobility inefficiencies. Finally, we calibrate the model on the French economy and show that implementing the optimal policy would account for significant increase in employment and productivity, through its impact on vocational training, occupational mobility and search frictions.