I have vague distant memories of mixing or cross breeding berries or something similar, but I can't remember where.
Yes, I realise I'm answering my own question, but with good reason. I've spent the last six months trying to remember where I experienced this mechanic, so I'm putting this here so if anyone else is scratching their heads trying to remember such a thing, this will help. It was in fact from gen VI's 'X' and 'Y'. Putting certain berries next to each other resulted in one of the bushes having a third variety of berry due to cross breeding. https://bulbapedia.bulbagarden.net/wiki/Berry_fields_(Kalos)#Mutation