In Trouble Brewing, if an incorrect order is made, an "X" will appear after you click on the green button to serve the drink.
In the screenshot below the order calls for 1 sugar. However, 2 sugars have been placed in the cup. To correctly fill this order you need to use only 1 sugar.
Therefore, when the green button is clicked to serve the drink, an "X" will appear to indicate the ingredients were wrong.
If you attempt to fill an order that does not exist (there is no order slip for it), then you will see a red X when you click the cup to serve it. This means that the cup has been thrown out, since there is no order with those ingredients. If you make a mistake, you can click a cup to remove it anytime during the game. Once you place 1 sugar into a cup and start the order, the order ticket will disappear. Any order tickets that you see are ones you have not started filling yet. Please note, the sequence in which you make the orders does not matter.
Starting in level 5, there will be orders in the first room on the left, and the coffee machine is in the right room with the ingredients. You'll need to press the arrows to move back and forth between the rooms, viewing and making orders.