
Continuity mistake: When Ash realises that something happened with Pikachu, his bag is below the second-highest plank on the bench, but in the next shot, it is aligned with it. (00:01:27)

The Ghost of Maiden's Peak - S1-E20
Continuity mistake: When Ash asks, "You remember where we're staying tonight, right?" Brock's bag is on dirt, but in the next shot, it is on grass. (00:08:37)

The Flame Pokemon-athon! - S1-E33
Continuity mistake: When Ash says that he cannot ride Ponyta for Lara, his arms are outstretched, but in the next shot, they are at his sides. (00:08:30)

Pokemon, I Choose You! - S1-E1
Continuity mistake: When Pikachu uses Ash's Pokédex for facts on where Pokémon want to reside, Ash's right fingers are covered by it, but in the next shot, they are not. (00:10:19)
Pokemon, I Choose You! - S1-E1
Continuity mistake: When Ash and Misty turn to look at the flock, Misty is pointing her right index finger away behind her, but in the next shot, she points it in front of her. (00:16:41)

Pokemon, I Choose You! - S1-E1
Continuity mistake: When Misty turns to look at her bicycle, the left pedal on it is higher than the right one, but in the next shot, it is lower than it. (00:16:43)

Showdown in Pewter City - S1-E5
Continuity mistake: When Ash says, "That's a great idea!" Flint's right arm is turned towards his right side, but in the next shot, it is turned forward. (00:10:12)

Continuity mistake: When Pikachu agrees after Professor Oak says, "That's the spirit!" he is on one tile, but in the next shot, he is on two. (00:11:08)

The Water Flowers of Cerulean City - S1-E7
Continuity mistake: When Brock asks, "Shouldn't you head for Cerulean City Gym?" a street lamp goes from being present to being absent. (00:06:14)
Continuity mistake: When Ash first uses his Pokédex, the top two fingers on the left side are together, but in the next shot, the bottom two fingers on the side are together. (00:07:35)
Continuity mistake: When Ash agrees to the fact that this is his first time at a Pokémon League, the front part of his hat does not go past the visor, but in the next shot, it does. (00:02:39)

Continuity mistake: When the phone rings, it is six centimetres away from the lamp, but in the next shot, it is fourteen centimetres away. Also, the cabinet is three centimetres away from the top of the planks directly above it, but in the next shot, it is six centimetres away. (00:04:34)

Continuity mistake: When Officer Jenny is surprised when she notices Ash, her grip on the intercom changes between shots. (00:02:11)
Answer: There are a lot of fan theories around this, but the most common theme is that Pikachu had another trainer before Prof. Oak, who gave him to Ash. Pikachu didn't appreciate being handed over.
Ssiscool ★