I worked for a motor parts shop in my ill-spent youth. One of the lines we sold was galvanised trailer wheels. The galvanising left rough residue which we removed with a file, creating quite pile of zinc fragments over time.
One day the foreman mechanic at local VW dealership was working on a Beetle engine and told a green apprentice it was necessary to put a teaspoon of compression powder in each cylinder of an air-cooled engine before putting the heads on, but they had run out. He sent the boy to us with a properly made out order for a pound of compression powder.
Instead of letting him in on the joke, one of our lads scampered down to the tyre changing area and filled one of our paper bags with zinc fragments. He sealed the bag with our branded sellotape and gave it to the apprentice with an explanation that we only had half a pound in stock but would get more later. He then solemnly filled out an invoice and sent the apprentice on his way.
We all fell about laughing, but the joke was soon on us: when the apprentice arrived back at the dealership it was smoko time (morning tea) and all the workshop staff were in the tea room. Being a keen sort, the apprentice decided to save the foreman some time and placed a teaspoon full of zinc powder in each cylinder of the engine his boss was working on, then put the remainder, still in our bag, onto the workshop supplies shelf. Joining the others for smoko, it did not occur to him to tell the boss what he had done.
The boss buttoned up the engine and test started it. Yes, it did, yes, he did and yes, we were. I've never seen a bloke so angry.
It all seems funnier now than then.