I've Got Batman in My Basement - S1-E20
Revealing mistake: Sherman puts down the Jr. Detective box to try to get back the stuff the bullies grabbed. He sets the box on one of the steps, but the way it is drawn defies gravity. (00:03:40)
I've Got Batman in My Basement - S1-E20
Revealing mistake: Penguin grabs the umbrella from the floor. For a moment you can see two umbrellas, and in the next shot the top with the smashed rotating blades is fixed. (00:19:50)
I've Got Batman in My Basement - S1-E20
Revealing mistake: The thief kicks away the canister bird seed was falling on...and the bird seed keeps landing in the same spot, which at this point is thin air. (00:05:10)
I've Got Batman in My Basement - S1-E20
Revealing mistake: The two kids grab a couple of items from Sherman's detective box, but the items never go away, a duplicate is spawned instead. (00:03:30)
Answer: She escaped off-screen and created a copy of herself to fool everyone.