This could be with anything, including trading with other people, playing with others on a server, etc. This is the list that I've gotten:
When I say specifically "Pure only" and someone adds me to offer in items.
When someone adds me, says that they want to offer on my item, offers less than the current C/O, and then says I'm a highballer when I cancel the trade.
Quickbuying trades that buy less than other people (I.E. Craft hats for 1.11 when there are multiple trades for 1.22-1.33).
When people call me a jacka** for rejecting their offer.
When people say they have an item for this much, I add them, and they say "Could you add a bit more?".
People who think that painting an item makes it still retain the full paint value.