Visually deceptive “proofs” which are mathematically wrong

Related: Visually stunning math concepts which are easy to explain

Beside the wonderful examples above, there should also be counterexamples, where visually intuitive demonstrations are actually wrong. (e.g. missing square puzzle)

Do you know the other examples?

Answer

The never ending chocolate bar!

Visual

If only I knew of this as a child..

The trick here is that the left piece that is three bars wide grows at the bottom when it slides up. In reality, what would happen is that there would be a gap at the right between the three-bar piece and the cut. This gap is is three bars wide and one-third of a bar tall, explaining how we ended up with an “extra” piece.

Side by side comparison:

Visual

Notice how the base of the three-wide bar grows. Here’s what it would look like in reality1:

Visual

1: Picture source https://www.youtube.com/watch?v=Zx7vUP6f3GM

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