Got a few over the years but the first one that jumped to mind was when was in my first year as an apprentice mech.
Those who know about apprenticships know the first year is spent on the end of a brush, making tea and going for the sandwiches as well as being the general rubbing rag in the workshop.
Anyway this one day i was told to go for the sandwiches and put the tea urn on to boil before i go.
I was in a rush and didn't want to lift the urn from one room and carry it to the tap to fill (yes it was a bit of a s**t hole and nobody had even heard of health and safety).
So spotting an watering can i had an idea, fill the can and take it to the urn. 

 brilliant.
There was already water in the urn, 

 even better.
I filled the urn and went for the sandwiches, got back and the kettle was almost boiled, thought that's close enough and made the teas.
All the mechs were sat around a small table eating and drinking the tea.
While i sat with a bottle of Irn-Bru smiling at a good job well done.
First one then two and so on till most of the mechs were in the bog throwing up. 

It turned out the watering can wasn't full of water, it was full of petrol. 

I'd had a cold and didn't smell it.
Needless to say i was taught a lesson 

I was stripped bollock naked covered in Swarfega and dipped in a degreasing tank, then i was thrown out the back door and left to try and get back in without being seen still starkers and covered in all sorts of grease and oil.
Happy days.
 Rab.
VIVE LA FRANCE