Not wishing to denigrate the REME who I'm sure do a wonderful job but years ago at my 'resident' AAC Squadron at Farnborough (originally 664 but later re-numbered 656), a pilot walked out to a Sioux and called for startup. I saw his rotors turning then occupied myself with other traffic.
A few minutes later there was a phone call from this pilot apologising for not telling the tower he had shut down again because 'I was a bit annoyed because when I started up, (it had been for an air test after some maintenance) I discovered the REME had fitted the rotor blades upside down'!