For this project, there were several sources of failure. For starters, two of the custom boards were DOA. It turned out the manufacturer used an NPN transistor in the output circuit when a PNP was called for. I had to contact the manufacturer and arrange for replacements. Not a major problem, but it took several days for the supplier to provide the replacement parts. Then, there was operator error. I flipped a green and blue pair when wire-wrapping the LCD to the Arduino on one of the simulators. With a wire-wrap tool, this was an easy quick fix.
Slightly more problematic was my MakerGear M2. It performed superbly when printing the circuit board holders in PLA, but when I switched to ABS plastic, output quality became erratic. After a bit of experimenting, I found that increasing the platform temperature a few degrees solved the problem. As I've learned on several models of 3D printers, these problems are to be expected. Still, each failure cost time — each component required two to three hours to print. As it was, the printer was cranking nearly 24/7 for a week to make the delivery deadline.
In the end, I delivered 10 units on time. Within a week of that, I had repaired the additional three units, which I maintain for backup and quick replacement. Based on my failure rate, I'd say that a proper maker's dozen is 13.5 units. That is, if you need to make a dozen relatively complex devices, order enough spare parts for 1.5 additional units.
You might be wondering how any business could succeed with such a high failure rate. After all, if one in 10 or so consumer electronics products failed, then there wouldn't be much of a consumer electronics industry. A difference between DIY and commercial electronics is that DIY components are often seconds and the overall design specifications shift over time.
Consider the 3D printer, for example. After the first print of a given component, I inevitably modified the model to improve it. Adding a brace to a thin wall, moving a mounting hole, or simply changing the color of the PLA filament has risks. In addition, not all PLA is created equal. I've found that the inexpensive bulk PLA on eBay simply isn't as good as the more expensive PLA filament from the print manufacturer. I don' know if it's the chemical composition, the diameter, or variation in some other parameter, but I do know that the PLA from MakerGear produces consistently better prints.
If you're building one-offs, you might not notice this relatively high failure rate. It's worth considering if you're building a number of units for, say, a club or classroom. Plus, when it comes to DIY, it's not really a "failure rate" but a learning opportunity. After all, you're not going to learn much if your experiments always work. On the contrary, if you're DIYing is 100% successful, then you're not pushing the edge hard enough. Good luck with all your DIY projects. NV