In theory, something like a technical job that requires extensive training. I always see reports about a lack of training for high tech trade / manufacturing jobs, as they require up to 2 years of training, but training slots are limited. You might get companies to pay the training if they could be assured the person would not run to a competitor. But even that should be agreed up front, with a limited term (eg we will train your two years, then pay you $X, and you will be subject to a noncompete for four years)