#45 You use Pokemon that were common in DP UU thinking that they're automatically still good.
This one's not as relevant these days but I still see it. A whole lot changed in UU with the shift of the last generation. Pokemon like Milotic, Registeel, Arcanine, Ambipom, and Mismagius used to be the cream of the crop in the day. The metagame centered around threats like these. Nowadays, though, they have a lot of things to compete with. They only work if you need very specific niches, but often you're better off with something else. This is basically a specific example of the advice "pick the best things for your team", as opposed to what you like or what you think is cool (I know Milotic is pretty and Registeel s your favorite legendary and whatever, but there are better bulky waters usually and Bronzong is awesome).