I read an interesting paper of QEC with only two extra qubits.
Here are some contexts of the paper:
"The stablizer is $XZZXI$. If the second gate fails and introduces an $IZ$ fault, then this fault will propagate through the subsequent gates to become an $IIZXI$ error on the data."
Since the stablizer on the second gate is $Z$, how could it introduce the $IZ$ fault? $IZ$ is equal to $Z$, isn't it? Why the subsequent gate becomes $IIZXI$?