Only time I've come across a trainer was during my gym induction and due to his lack of instruction, I got injured even though I'd never had an injury prior to that.
As Titan points out though, there are a percentage of trainers that may have only done a very brief or the most basic course offered in order to have the minimum requirements necessary for their job at a gym/fitness centre; I suspect that is probably the case with the trainer that I had.
If I'd done my own research about the mechanics of the machine I was using, I doubt I would have got injured. Do your research, don't leave your body entirely in the hands of someone else.
36.5 kg /80.3 lb Middle-Finger DL (right hand)...