I always thought that if I ran my own lab, I would have rotation projects be to replicate a recent result from the lab. Then, a lot of the infrastructure and support would still be there, so it would be pretty clear if the fault lay with the experiment. Plus, it would reinforce the notion among trainees that the point of science is to be replicated, and that the hard part of doing something novel is figuring out what to do, not actually doing the work.