I'm sure people have seen this in the past, I just noticed it about a week ago...
When you trade with another player... If you have a full inventory and then take things out of a backpack you are carrying, and put the items into the Trade...
This means there's not enough space in your Inv to put things back if someone cancels the Trade. Therefore each time I tried this, I lost whatever items I was trading and the overflow box that's supposed to show up never did.
This has happened a few times, I've lost entire suits of armor because of this glitch.