With the bar see-through, the black line runs straight on into the red one.
Straight, bent or tilted?
Poggendorff illusion
Which line on the right, red or blue, continues the black line?
The red one. Most people pick the blue. “Prove it” makes the bar see-through.
Why? Your brain misjudges the angle where the line meets the bar, so it expects the line to come out in the wrong place.
Picture: Fibonacci, CC BY-SA 3.0 · Wikipedia(opens in a new tab)