Any Trade-O-Mats that share a side (touching top/bottom or sides) will treat adjacent Trade-O-Mats as valid inventories, such as a chest would. That means that if one Trade-O-Mat offers an item, and a touching Trade-O-Mat wants that item for a trade (or vise-versa) it will grab it, and complete its trade. This can and will cause bad looping of trades and loss of items for players. Those of you with touching Trade-O-Mats will probably see lock signs on them. They were added by a Moderator to protect you and your customers. Ask a Moderator (see this) to remove the Trade-O-Mats when you have the time to update your shop. All Trade-O-Mats need to be spaced one block apart until this bug is fixed.