[fadetree]

client a opens trade window
client a tells server to tell client b to open trade window
client b opens trade window
player a puts item in trade window
client a tells server who tells client b about it
client b acks
player b hits accept
client b takes item into inventory
client b tells server okie dokie
server TRIES to tell client a about client b now having the item but
DDOS fail fail fail
client a never gets told the trade finished, so it never removes the item
voila - DUPE

Rogean - you could look for people just standing around swapping items a lot back and forth at the usual ddos times