It wasn't a complicated bug. The trade interface accepted item quantities for stackable items: gold, bank notes, runes. A modified client allowed you to send a packet of offering into the trade a non-stackable item with zero quantity. Then at some point during the transfer, the quantity for non-stackable items gets ignored, because it should always be one, turning a zero into a one!

https://archive.fo/DafO7 is an archive (of an archive) since I can't find the original page.