= Timed bit flip during repetition-code syndrome extraction
{title2=$s=(e_2\oplus e_3,e_1\oplus e_2\oplus f)$}
Suppose the first ancilla records parity $q_2\oplus q_3$, then a fault of value $f$ flips $q_2$, and the second ancilla records parity $q_1\oplus q_2$. For incoming bit-error pattern $e$, the observed <error syndrome> is the displayed pair, while the final data error before recovery is $d=e\oplus(0,f,0)$. When $f=1$, the observed syndrome differs from the actual data syndrome by $(1,0)$. Applying the usual minimum-weight recovery leaves a nonzero syndrome, so the state is outside the code space even when the channel itself made no error. This is a timing-dependent failure of this simple extraction circuit, not a general obstruction to fault-tolerant <quantum error correction>.
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