If he connected series instead of parallel or if he connected with reverse polarity, the capacitor on the controller would blow - normally with a bit of a bang and some horrible smoke, so it's pretty obvious at the time. You should be able to see that when you take the controller off. The components are potted in silicon, but I think you'll still see and smell evidence of it.
Controller is the pod behind the chainwheel that's held on with three screws.
Thanks d8
I'll let you know when it's off