Amplitudes are not probabilities. They have signs, and in general complex phases, and they *add*. When two paths lead to the same outcome, their amplitudes can reinforce or cancel. That is interference, and it is the one thing a quantum computer does that a coin cannot.

Two Hadamards in a row do nothing: the paths to |1⟩ cancel and the paths to |0⟩ add up.

```qasm
OPENQASM 3.0;
include "stdgates.inc";
qubit[1] q;
bit[1] c;
h q[0];
h q[0];
c = measure q;
```

This program measures 0 every time. A phase in the middle changes which paths cancel. The phase itself is invisible to a measurement made right away, but a later gate that mixes |0⟩ and |1⟩ turns it into a different outcome.

The engine's critique flags gates that cancel, such as `h q[0]; h q[0];`, as a smell: `wai-quantum critique` names it with its lines, and the Studio marks it on the drawing.