I'm thinking there's no way this was a mistake.
http://www.rte.ie/sport/racing/2017/0727/893432-controversy-yarmouth/
Trainer saddles up one of his 3 year olds as a 2 year old, okay that can happen I guess. But then it goes and wins... at a price of 50-1.
They were caught by some kind of testing which appears to be routine so you would think they wouldn't expect to get away with it, but as I said the fact the horse won at such big odds it seems unlikely to be just a mix up, had to be planned.