They show a Geodude in the pokémonrap at the end when they say Graveler and the name display says Graveler.
When Brock calls out Onyx to battle Ekans and Koffing (near the end), Jessie says, "Let's show them that size doesn't matter! Ekans, underground!" As Ekans begins to dive underground, both Ekans' and Arbok's voices can be heard at the same time. There should be no Arbok voice because there is no Arbok.
When Ash and the gang first encounter Team Rocket for the first time, Brock seems to already know who they are despite the fact that this is the first time in the series he actually meets them.
Pikachu is able to lift Clefairy with no problem despite the fact that Clefairy is supposed to be four pounds heavier than Pikachu.