The other one noted if you don't specify the density of plastic for bags, or paper for bags and packing, you get clingfilm thinner than you thought existed, and paper which is almost tissue in its weakness. You don't even get boxes to put the boxes in, if you don't specify boxes to be delivered in boxes. So now wrapping a pallet becomes a nightmare if they don't stack. And if you don't specify how many to stack, and how to pad the stack, they won't do unit height stacking if it costs labour time. Your risk.
Some of this like the casting mistake, or the knob thing, could happen anywhere and you have to be close to final manufacture spec to find out e.g. the metal coating impinges on the knob at the free space you specified, because your test rig didn't have powder coating. Or, that a design feature you need like the light entry holes, is used by the casting engineer as pour points because it looked like you'd specified mould pour points not functional holes.
But other things like "yea, you didn't spec how long to make the tails so we cut the tails as close as we could" is just the cheapening above: if you don't SAY its a 10cm tail for the connector, it will be 2cm, if saving 8cm of cable saves money for them.
I've read some stuff which says the cost of 5 SBC boards with pre-applied SMD is now so low, you might as well order 5 so you get at least 1 which works. That means they will wind up working out your tolerance for failure, and produce goods to meet that: if 1 in 5 is viable, thats what they'll target.
They'll make this amazing Remote Control Car, with good suspension, a battery that lasts half an hour, plenty of power, and just all around amazing. But then it'll break after a day because somebody saved 1/20th of a penny by speccing this impossibly thin wire the thickness of a human hair to hook that powerful battery to the powerful motor and inside the remote.
They could have used actual wire-sized wire and had the most amazing product ever, for roughly zero more cost. (Possibly less, since surely it must cost _more_ to manufacture and solder micron-diameter wiring). It just makes no sense.
I ordered cups and did specify the thickness (based on a reference) of the plastic but didn’t specify how thick the boxes they shipped in should be. Guess what happened!
That's the thing that makes no sense to me. Wouldn't it work better for everyone involved, including the manufacturer, if they come back with a "here's exactly how we interpreted your spec and what materials we're using, including cases where we picked something you left unconstrained", and a corresponding price for using those materials, with the understanding that if you want different materials you get a different price, before they do any manufacturing whatsoever?
But a whole lot of manufacturing relies on the OEM not just for the production but also some of the engineering. They probably don’t communicate to you all of the little parts you didn’t spec because they don’t think you want them to.
A friend of mine worked for a major vacuum company that has 3 well-known lines. The very high end line is manufactured in the USA. The mid range line is fully designed here but manufactured there.
The low end line they basically tell a Chinese OEM what it should look like and certain parts of it and let the OEM fill in the blanks. OEM makes vacuums for other brands too, they know what size wires to use where, why waste money on American salaries speccing all the tedium?
OP is making something fundamentally different than what exists in some ways. It’s a lamp, but obviously it has some considerations other lamps don’t.
Also hardware (as OP pointed out) has long development cycles during which prices can actually fluctuate significantly. You don’t want to lock in cost plus pricing or you’ll end up buying components from their sister company at astronomical prices. You might not have time to source 100 different parts from Chinese OEMs yourself. Etc.
The whole thing is a complex guessing game. I’ve only manufactured approximately the simplest objects imaginable (stuff like gaskets and cups) and found it to be far more complex than I thought it would be. Manageable but something like a lamp I could see being a lot of unexpected work. I did not see, for instance, how much effort I had to put in speccing out the packaging for simple objects but then I found myself repacking things here.