My biggest one was probably taking the wrong trailer and driving 2-3 hours to a store only to find out I have the wrong trailer.

Needless to say, they were not impressed.
edit:
Oh, I had another one.
I was asked to drive a rigid. So I threw bag and phone in the truck and went to get the paper work. I jumped back into the truck after getting my paperwork only to realise all my stuff had disappeared. I was so confused.
I searched everywhere in the truck for my bag and phone. After searching for my stuff in the waiting room and the truck for about an hour I realised I jumped into another truck after getting my paperwork.
(All the rigids are all lined up in a row and look identical
