Character mistake: Often times the game gets the weather wrong. Usually, it will show there is rain in the game even if it's not raining where you are. This can often be due to rain in a nearby location, but it reads the local weather wrong. Or it can be raining IRL but it doesn't show in game, which can cause you to be unable to evolve rain requirement Pokémon.
Revealing mistake: The Alolan variant of the Exeggutor is portrayed as being so tall that its head is usually off screen above the camera, but when bending down for attacks you can still see it's head. If you defeat a gym while using the Alolan Exeggutor however, the victory screen will have it's head cut off much lower down and just disappearing into the void where it could still be seen otherwise.