As an FYI, many of the X-Ray machines (especially the Smith-Heimann) calibrates itself by backing the belt up then proceeding forward. This happens often especially if a bag is cut on the screen. This is not controlled by the operator so when putting your laptop on the belt make sure the belt is moving forward and don't just load it behind a bag and walk away.
The Rapiscan series does have a steel table right before the belt to stop this. It's really nice, in that it won't let a item be pushed off when the machine goes in reverse like it does.
Problem is, a lot of people place their items on that table then leave it there and proceed through the WTMD. And depending on the positioning of the WTMD in relation to this table, the screener may not see that the passenger has done this.
So a little advice, if you encounter a x-ray with a steel table right before the belt, make sure your items are on the moving belt before you proceed.