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.
The objects were order of magnitude simpler than in the post (no electronics and no plastic, only metal) so maybe that doesn't compare, but I never had any bad surprise from any supplier, including packaging (which can be quite complex and involve several providers), etc.
Everyone will gladly send you samples (for free!) and prototypes of what you imagine (usually at cost) and if you're explicit about what you want and validate each step before the next, everything goes well.
Eventually I moved on to other things for mostly bureaucratic reasons; selling objects in Europe is an administrative nightmare that's simply not worth the hassle.
But the manufacturing part was not just smooth -- it was the best part of the experience.
(And I never left my town and never even talked to anyone over the phone: the primary means of communication was email.)
Edit: why would anyone downvote this, and so fast? If anyone thinks I'm being insincere, I have proof! ;-)