When you try to print your scanned $20 bill, you discover the problem. The fact that the scanner can capture such detail does not mean that your printer is ready to replicate it. If you try printing on a normal inkjet printer, it comes out looking all wrong to the naked eye. The colors are off and the images look muddy.
You can see why it looks wrong when you put your new counterfeit bill under a microscope. For example, many fine details are completely lost:
And the light-colored hexagons turn to a brighter shade because the printer can't reproduce lines that are fine enough or light enough:
Compare these two images to their corresponding images from the previous section.
It's obvious that they are not even close. You can actually see what is happening here -- the printer cannot replicate the fine lines exactly. In the case of the hexagons, the blobs that the printer ends up producing make the light color of the original bill a much brighter shade. This effect is the very reason for imprinting the lightly-colored hexagons on the bill in the first place -- they make the bill harder to replicate with current printer technology.
To some extent, these printer problems can be remedied with a better printer. However, even the best printers lose some of the detail. If a person were to look at the inkjet printer's counterfeit bill with a magnifying glass, it would be obvious that it is a counterfeit. But to the naked eye, a good high-resolution printer can produce a counterfeit bill that looks pretty close to the real thing.
To create an actual bill, you are going to have to test-print your scan a number of times and adjust the color to get the overall tone right. You are also going to have to scan the back of the bill and practice aligning the front and back sides to get a realistic two-sided bill.
If you are careful with your technique and if you have a good enough printer, you will end up with a bill that, although not perfect, looks passably good to the naked eye. It won't have the color-shift ink and it will look a bit muddy in places, but as long as the recipient doesn't look at these details very closely, the illusion will be "good enough" to work.
The easiest way to get around the visual imperfections of your counterfeit bills is to spend the bills in situations where the exact visual details are hard to see. For example, you might try to hand your fake money to a waitperson in a darkened bar or nightclub.
As soon as the person touches the counterfeit money, however, it will be obvious that something is wrong. That's because of the paper.