A mate of mine ended up in ground resonance in a Wessex because he didn't spot that one of the blade damper locks had been left in - he didn't spot the 2 foot long red tag attached to it because the engineer had helpfully wrapped it tightly around the lock so it was almost impossible to see at 4 am on a rainy Belfast morning.
I was taking the new Sqn EngO flying in a Wessex in Cyprus and he helped on the walkround by climbing up the port side to open the transmission decking as I did the same on the starboard side. As we faced each other acroos the decking I noticed an 8 inch long paintbrush wedged amongst the control runs from the mixing unit to the jacks. Needless to say we didn't go flying and the engineers quickly discovered it was not a good idea to embarrass their new boss!