You are a detective investigating a crime scene. On the floor lies dominoes - someone arranged them and induced a toppling chain-reaction. The policemen are collecting the dominoes to identify the suspect's fingerprints. You sidekick, Dr Watson, claims
"We don't have to check all the dominoes. First, pick a random domino and keep looking back at the domino it falls under until one that has no domino topples on. The suspect must have manually pushed it, hence will contain his fingerprint!"
Here are three configurations of dominoes. Which of them fails Watson's method?
[U shape picture here]
[Y shape picture here]
[O shape picture here]