A marmalade is when you ask your mate to get you something from the shop and they deliberately bring back the last thing you would have asked for.
Mate: Do you want anything from the shop?
You: Nice one. Yeah, a Snickers?
Mate comes through the door with a jar of marmalade.
You look at him like, 'what's wrong with you mate?'
Straight-faced, he goes, 'Oh, Snickers. I thought you said marmalade.'