The engineer used sight and touch to match the bolts and, erroneously, selected 8Cs instead, which were longer but thinner.
Correct. This was a slip or - bad execution. The engineer’s plan to use 7Ds was not a good plan. Actually, it was a bad plan. This plan was the result of a violation, but as mentioned previously, in aviation single error generally does not cause an accident. The 7Ds were in place previously and – even though not approved – held the windscreen. The engineer poorly executed his plan and used the thinner 8Cs. A slip, which makes the second error in the chain.