By understanding the processes in which MTG cards are made, we can understand how the errors we see in our packs come to exist. Misprints can happen at any stage of card production so this simple overview will give you a better idea of the step by step processes that go into production, and what types of errors happen at each of those stages.
Step 1: Design
Ok, so this one seems logical right? Before we can manufacture the cards we need to design them. I'm using this step, though, to cover pretty much everything that happens before someone hits the big red button labeled 'print'. The important thing to note for misprint collectors is that this is where the source of all human error misprints lives. This is largely due to the many printing variants that need to get designed. We'll often see minor (or major) mistakes between these variants. For example; Traditional Chinese Foil M13 planeswalkers might be missing their loyalty counter number or Italian 5th Editon Breeding Pits have no casting cost, etc. For a list of known human error misprints check out this link, and this one too.
I'd also like to mention that part of card design is also designing new looks for never before seen cards, game mechanics, and other changes. Test prints are developed and printed to see if they're what WotC wants for their new products. Tests prints are made for things like new card frames, new foiling methods, holostamps, and more. Check out the librarities website for a list of all known test prints.
Step 2: Printing
Magic cards are printed in a process called offset printing. In offset printing each of the 4 printing colors (CMYK**) are printed separately from one another. Each color being printed separately gives us room for errors to happen at each color. Single or multiple color channels can run low on ink, have too much ink, have something obstruct the channel (like a splotch), or have the rubber rollers not properly cleaned between passes resulting in all sorts of neat misprints, which I cover in the beginners guide to misprints. Here's a video that describes offset printing better than I could:
**Note: Foil cards get an extra layer of ink applied to them before the CMYK layers called the underprinting layer. The underprinting determines which parts of the card allow the foil to show through and which parts are masked. These masks are created by layout artists and are greyscale representations of which parts of the card should show how much foil. White represents no underprinting (100% foil) and black represents full underprinting (0% foil).
Step 3: Cutting
Once the sheet is printed, it's time to make cards out of it. This is done in two separate cuts. It should go without saying that if the sheet gets a little shifted during this time, that this is how we get miscuts. There's really not much else to say here that the video below won't show you!
Step 4: Corners
When the cards are cut from the sheet, the corners are still square. The cards are put through a die press which neatly cuts the corners. Interestingly enough, you can tell which side of the cards are facing upwards when the die press makes the cuts by taking a close look at the corners. The video below doesn't specifically refer to magic cards, but it does show you how printing at Cartamundi works!
Step 5: Packaging
The last step of the process is packaging the cards into sealed products so they can be shipped out and sold to you. This includes wrapping them in boosters. In the process of sealing boosters closed, cards can get caught in the heat press and get crimped. Also, sometimes there are issues with the booster foil. There have been "miscut" booster packs, as well 'double printed' boosters, and even boosters with a tape-like covering that have been found in booster boxes.
I hope you've enjoyed this article on the manufacturing of MTG cards from a misprint collector's perspective. It's important to note that magic has been printed for over 20 years. In that time the technology and processes have changed slightly. Machines have been updated, and new print facilities around the world are now being utilized. There's no one way that all magic cards have been printed, but the general principles have stayed the same (for now). The key thing to continue to look for are new things in magic. Think about how the print process has to change if a new unique mechanic alters our perception of a normal magic card. There's always opportunities for new types of misprints!