Before berating mechanics, let’s not forget everyone, including experienced professionals, make mistakes occasionally.
I once got distracted when assembling a piece of equipment and missed fitting a trigger device (and packing the spares) simply because someone asked me a question which I stopped to answer then I missed a step in the process. That involved a german warship sailing 4 days back to port to pick up the missing bits.
It’s not the mistake but how they deal with the response that should define how good they are.