ok, so I was trading from my Ruby to my Emerald, I was trading a tentacool holding a water stone from my ruby and a Shroomish from my Emerald, I traded, everthing went as planned but then when I came out of the trading room I switched off both of my games without saving after exiting the room cause I assumed that I didn't have to save. Then when I switched on my Emerald, Shroomish was still there and there was no tentacool with water stone and then I switched on my Ruby and there was no tentacool with water stone either, there was the shroomish.... what the heck just happened here? a copy of shroomish ? where did the tentacool go? mind blown ._.
Thats called a duplication glitch. To keep Tentacool you need to save. However, in Gen V, you dont need to. Hope I helped!