1) Looking for the lowest price
If you purchase something below market price, you are taking very high risks. Either quality will not be up to your standard, or you are about to get scammed.
2) Arranging shipment without quality inspection
Once a production batch is on a boat, it’s too late. Verify quality yourself or pay for quality inspection services. You need to do it systematically, at least for the first 5 shipments, after that, you can do random skip-lot quality inspections.
3) Not keeping two weeks cushion for schedule
There will be delays, with a certainty comprised between 30% and 90%. So, plan for it. Bonus: even worse than failing to plan for delays, is pushing the factory to reduce production time.
4) Paying in full while one still needs leverage
Some purchasers get a really good feeling about a supplier and agree to wire 100% of the order amount in advance. Then the manufacturer has no incentive to hurry up or to produce up to the standard. If quality issues are uncovered (and that’s only if they allow for an inspection), the factory might refuse to rework the goods.
5) Hoping an unsatisfactory manufacturer will get better over time
Based on a recent survey, it looks like it is a bad idea to give a second order to a factory that just produced substandard quality. Rather than rolling the dice (when the odds are 90% against you), nurture a backup manufacturer.